Category Archives: Engine

Cleanest SUV Engine – Volvo XC90 T8 Twin Engine

1_ 148026_The_all_new_Volvo_XC90_D5_Drive_E_engineVolvo’s XC90 Twin Engine has recorded CO2 levels of 49g/km, making the vehicle, its makers claim the world’s most powerful and cleanest SUV.

The improvement over earlier announced figures was achieved thanks to the continuous innovation cycle at the Swedish company. The result of further tuning is a drop of 10g/km from the vehicle’s initial prognosis, for up to 2.1 l/100km (134.5mpg) fuel economy, while also adding horsepower.

“We have been working hard to earn our competitive edge and to give our customers the ultimate combination of performance and low fuel consumption,” says Peter Mertens, senior vice president of research and development at Volvo Car Group.

“Our Twin Engine technology has enabled us to build on our heritage of efficient powertrain development in a completely new way. Thanks to our new scalable product architecture, and our world class four-cylinder engines, we have a clear and leading position.”

The XC90 T8 delivers 41.8km of pure electric range, 0-100km/h in 5.6 seconds, and combined power of 412ps and 640Nm.

The vehicle has five different driving modes. The default setting is Hybrid mode, in which the car automatically alternates between drawing power from the 2-liter, 4-cylinder Drive-E engine and the electric motor to deliver the best overall fuel consumption.

In Pure Electric mode, when fully-charged, the high-voltage battery serves as the car’s sole energy source, powering the electric motor over the rear axle. The XC90 T8 has a range of up to 41.8km using just electricity, which covers the total distance many people drive in one day. The regenerative braking system makes the vehicle efficient in the stop-and-go traffic of city environments, and if more power is needed, the Drive-E combustion engine starts up automatically.

In Power mode, drivers get the combined performance of the combustion engine and the electric motor. On start-up, the SUV takes advantage of the electric motor’s response and instant torque curve, while the combustion engine gets up to speed. This combination offers better torque at lower revs, equivalent to that of a large displacement engine.

AWD mode offers constant all-wheel drive on demand. The advantage of being able to select AWD manually is that the driver can use it when needed, or choose to save energy for later.

If the battery is charged, Save mode allows the driver to ‘freeze’ the battery level and save it for later use with Pure Electric drive. On the other hand, if the battery is low, the driver can use the combustion engine to charge the battery to a certain level for later use with Pure Electric drive.

Homogeneous Charge Compression Ignition Engines


Homogeneous charge compression Ignition engines uses best of both SI ( spark ignition ) & CI ( Compression Ignition) engines. In this type of engine fuel is injected into intake air & the complete charge is compressed to a point auto ignition.This ensures the better combustion than Stratified charge compression ignition. The process of combustion however remain same , density and temperature of air is raised to auto ignition temperature of fuel , but in SCCI engines fuel is injected just at the end of compression stroke whereas in HCCI engines fuel is injected along with intake air.

with the advent of stringent emission regulation its becoming more & more difficult for engine manufacturers to meet the legislation requirements. HCCI engines have potential of being very fuel efficient and produce low emissions. HCCI can operate on almost all conventional as well as alternative fuels. HCCI was a concept from long but recent advancements in sensors incorporation in engines and electronics over riding the mechanical systems its seems HCCI can be seen in near future operating inside the vehicle hoods. HCCI engines has potential to be low cost due to low pressure operations involved as well as the reduction in after treatment system requirement considering the low NOx emissions.

It becomes extremely difficult in HCCI engine to control the ignition process. As in case of SI engines it is spark controlled and in case of CI engines it is controlled by fuel injection , in an HCCI engine, however, the homogeneous mixture of fuel and air is compressed and combustion begins whenever sufficient pressure and temperature are reached. This means that no well-defined combustion initiator provides direct control. Engines must be designed so that ignition conditions occur at the desired timing. To achieve dynamic operation, the control system must manage the conditions that induce combustion.


  • Elimination of throttling losses.
  • High compression ratios
  • Shorter combustion duration (No Flame front, so distance to travel)
  • HCCI engines can operate on Diesel , Gasoline as well as alternate fuels.
  • Absence of Throttle loses , makes the engine further efficient.


  • No over control over ignition time
  • Cold start is poor.
  • Hydrocarbon & CO emissions is higher
  • High in-cylinder peak pressures may damage the engine.
  • High heat release and pressure rise rates contribute to engine wear.
  • HCCI engines have a small power range, constrained at low loads by lean flammability limits and high loads by in-cylinder pressure restrictions.


Saturn Aura was reported to be tested with HCCI engine.

Apart from Saturn , Mercedes & Volkswagen is also testing their HCCI engine vehicles which is going to hit market likely in 2015.




What is Automated Manual Transmission ?

Automatic Manual Gearbox Clutch actuator

In a conventional manual gearbox, a set of cables or a link usually operates the gearbox in a two-step process. If the gearbox is cable operated like for example in a Tata Nano, two cables do the process of selection and engagement of the gears. The selection cable is actuated when one moves the gearshift lever from left to right or vice versa. The engagement cable is actuated when one actually shifts into one of the gates to engage a gear. Similarly, a link operated gearbox, like the single-link shifter mechanism found in the old Maruti 800 works on a similar principle. In the case of the 800, the link rotates for the selection process and moves longitudinally to the centreline of the car to engage a gear. Simplifying the shifter process, an Automatic Manual Transmission, or AMT essentially negates these mechanical linkages by replacing them with electromechanical individual devices that work off the engine management and transmission management control units.
The electronically controlled clutch actuator uses an electric motor to operate the clutch instead of a conventional pedal and cable operated setup. In the clutch actuator, the rotary movement of the electric motor is transferred via a set of gears to a linear movement that is needed to engage and disengage the clutch via the pressure plate. The clutch is operated via a release lever and bearing which then helps to engage or disengage the gears in the transmission. ZF Sachs, the pioneers of the AMT gearbox claim that this setup not only simulates clutch-less driving for the driver, but also increases the life of the clutch components while cutting down on emissions and increases fuel economy.
 In an AMT, the electromechanical or sometimes hydraulically operated actuators take over the clutch and shift action, but do not take away driving pleasure. This is predominantly due to the fact that the driver can still choose to manually engage a particular gear through either a shifter lever or a steering mounted paddle shifter. With their optimised shift points, AMTs also help reduce torque interruption to a minimum between shifts. As an array of sensors register and convey all key information through a transmission control module, the system can automatically adjust shift points and control the clutch according to driver based input. The AMT’s ECU can also intervene to improve safety by automatically interrupting drive flow to counter the risk of skidding if the vehicle loses traction.
Apart from giving the driver the pleasure of driving an automatic car, the AMT system was essentially developed to automate manual transmissions in vehicles that cannot have a conventional automatic transmission. The AMT can also help in cars where a conventional automatic transmission or a Continuously Variable Transmission (CVT) is difficult to install due to cost, weight or installation space constraints. Apart from the Marui Suzuki Celerio that uses a Magneti Marelli, the Tata Nano facelift will soon sport a similar ZF developed AMT.

Diesel Variable Geometry Turbo

An alternative to the fixed geometry turbine is the variable geometry turbine. The benefits of variable geometry turbines over wastegated turbines include [Xin 2011]:
  • no throttling loss of the wastegate valve;
  • higher air–fuel ratio and higher peak torque at low engine speeds;
  • improved vehicle accelerations without the need to resort to turbines with high pumping loss at high engine speeds;
  • potential for lower engine ΔP (the difference between exhaust manifold and intake manifold pressures);
  • control over engine ΔP that can be used to drive EGR flow in diesel engines with High Pressure Loop (HPL) EGR systems;
  • a better ability to cover a wider region of low BSFC in the engine speed–load domain;
  • ability to provide engine braking;
  • ability to raise exhaust temperature for aftertreatment system management.
The idea of using a variable geometry turbine in a turbocharger dates back at least to the 1950s [Egli 1958]. Since that time, a number of different designs have appeared. Two of the more common ones are the pivoting vane and moving wall types, Figure 1 [Foulkes 1995][Merrion 1994]. Others include the variable area type, variable flow type and the sliding ring designs. These designs will be discussed in more detail in the following sections.

Figure 1. Pivoting Vane (left) and Moving Wall (right) Variable Geometry Turbochargers
1. Turbine housing; 2. Variable angle vanes; 3. Adjusting ring
There are a number of different acronyms that are commonly used when referring to turbochargers with variable geometry turbines. In most cases these are or have been trademarks that a particular manufacturer has used with reference to their product. In more common usage, a particular acronym can be used in a more general sense and not necessarily be a reference to a particular manufacturer’s product. Some of these acronyms include:
  • VGT—Variable Geometry Turbocharger (Cummins/Holset),
  • VNT—Variable Nozzle Turbine (Honeywell/Garrett),
  • VTG—Variable Turbine Geometry (BorgWarner and ABB)
  • VG—Variable Geometry turbocharger (MHI)
  • VGS—Variable Geometry System turbocharger (IHI)
  • VTA—Variable Turbine Area (MAN Diesel & Turbo)
In many designs, a variable geometry turbine does not include a bypass so the turbine must be capable of handling all of the exhaust flow from the engine while avoiding overboost and overspeeding the turbocharger. For a given engine power rating, this would imply a larger turbine swallowing capacity than that required by a wastegated fixed geometry turbine and comparable with that used for a fixed geometry turbocharger with no bypass.
The fundamental difference between a fixed geometry turbine and a variable geometry turbine is illustrated in Figure 2 [Schmitt 2008]. Compared to a fixed geometry turbine, the variable geometry turbine allows significant flexibility over the pressure ratio/flow relationship across the turbine and by extension, the engine ΔP. This flexibility can be used for improving low speed torque characteristics, reducing turbocharger lag and in diesel engines, driving EGR flow.
[chart] [chart]

Figure 2. Comparison of Fixed Geometry (BorgWarner KP39) and Variable Geometry (BorgWarner BV40) Mass Flow vs. Pressure Ratio
The peak efficiency of a variable geometry turbine occurs at about 60% nozzle opening. It is usually comparable to or a few percent lower than that for a fixed geometry turbine. However, efficiency drops off rather quickly as nozzle opening is reduced or increased from a mid-vane opening position, Figure 3 [Dinescu 2010].

How does Turbocharger Works ?

Picture of a IHI VF39 turbocharger from the tu...
Picture of a IHI VF39 turbocharger from the turbine exhaust side. (Photo credit: Wikipedia)

An engine is designed to burn a fuel-air mixture to produce mechanical energy. The mechanical energy then moves pistons up and down to create the rotary motion that turns the wheels of a vehicle. The more mechanical energy, the more power the engine can produce.

A significant difference between a turbocharged diesel engine and a traditional naturally aspirated gasoline engine is that the air entering a diesel engine is compressed before the fuel is injected. This is where the turbocharger is critical to the power output and efficiency of the diesel engine. It is the job of the turbocharger to compress more air flowing into the engine’s cylinder. When air is compressed the oxygen molecules are packed closer together. This increase in air means that more fuel can be added for the same size naturally aspirated engine. This generates increased mechanical power and overall efficiency improvement of the combustion process. Therefore, the engine size can be reduced for a turbocharged engine leading to better packaging, weight saving benefits and overall improved fuel economy.
Although turbocharging is a relatively simple concept, the turbocharger is critical to the operation of the diesel engine and therefore requires a highly engineered component. Our extensive experience in turbocharging technology and knowledge of engines combines for world-class design and manufacture of Holset Turbochargers, renowned for their durability, high standard of safety, and reliable performance that engines demand.
How does a turbocharger work?
A turbocharger is made up of two main sections: the turbine and the compressor. The turbine consists of the(1) turbine wheel and the (2) turbine housing. It is the job of the turbine housing to guide the (3) exhaust gas into the turbine wheel. The energy from the exhaust gas turns the turbine wheel, and the gas then exits the turbine housing through an (4) exhaust outlet area
Turbo Diagram(1)  The turbine wheel (2)  The turbine housing
(3)  Exhaust gas
(4)  E
xhaust outlet area(5)  The compressor wheel (6)  The compressor housing(7)  Forged steel shaft 
(8)  Compressed air

The compressor also consists of two parts: the (5) compressor wheel and the (6) compressor housing. The compressor’s mode of action is opposite that of the turbine. The compressor wheel is attached to the turbine by a (7) forged steel shaft, and as the turbine turns the compressor wheel, the high-velocity spinning draws in air and compresses it. The compressor housing then converts the high-velocity, low-pressure air stream into a high-pressure, low-velocity air stream through a process called diffusion. The (8) compressed air is pushed into the engine, allowing the engine to burn more fuel to produce more power.

Comparison of Spark Ignition (SI) and Compression Ignition (CI) Engines

  • The most prominent difference between Spark Ignition (SI) and Compression Ignition (CI) engines is the type of fuel used in each. In SI engines petrol or gasoline is used as fuel, hence these engines are also called petrol engines. In CI engines diesel is used as fuel, hence they are also called diesel engines.

    Here are some other major differences between the SI and CI engines:

    1) Type of cycle used: In the case of SI engines, the Otto cycle is used. In this cycle, addition of heat or fuel combustion occurs at a constant volume. The basis of working of CI engines is the Diesel cycle. In this cycle the addition of heat or fuel combustion occurs at a constant pressure.

    2) Introduction of fuel in the engine: In the case of SI engines, during the piston’s suction stroke, a mixture of air and fuel is injected from cylinder head portion of the cylinder. The air-fuel mixture is injected via the carburetor that controls the quantity and the quality of the injected mixture. In the case of CI engines, fuel is injected into the combustion chamber towards the end of the compression stroke. The fuel starts burning instantly due to the high pressure. To inject diesel in SI engines, a fuel pump and injector are required. In CI engines, the quantity of fuel to be injected is controlled but the quantity of air to be injected is not controlled.

    3) Ignition of fuel: By nature petrol is a highly volatile liquid, but its self-ignition temperature is high. Hence for the combustion of this fuel a spark is necessary to initiate its burning process. To generate this spark in SI engines, the spark plug is placed in the cylinder head of the engine. The voltage is provided to the spark plug either from the battery or from the magneto. With diesel, the self-ignition temperature is comparatively lower. When diesel fuel is compressed to high pressures, its temperature also increases beyond the self-ignition temperature of the fuel. Hence in the case of CI engines, the ignition of fuel occurs due to compression of the air-fuel mixture and there is no need for spark plugs.

    4) Compression ratio for the fuel: In the case of SI engines, the compression ratio of the fuel is in the range of 6 to 10 depending on the size of the engine and the power to be produced. In CI engines, the compression ratio for air is 16 to 20. The high compression ratio of air creates high temperatures, which ensures the diesel fuel can self-ignite.

    5) Weight of the engines: In CI engines the compression ratio is higher, which produces high pressures inside the engine. Hence CI engines are heavier than SI engines.

    6) Speed achieved by the engine: Petrol or SI engines are lightweight, and the fuel is homogeneously burned, hence achieving very high speeds. CI engines are heavier and the fuel is burned heterogeneously, hence producing lower speeds.

    7) Thermal efficiency of the engine: In the case of CI engines the value of compression ratio is higher; hence these engines have the potential to achieve higher thermal efficiency. In the case of SI engines the lower compression ratio reduces their potential to achieve higher thermal efficiency.

what is an MPFI engine ?

You must have seen cars with specifications which mention words like MPFI and CRDI or CRDE. To an automotive engineer or enthusiast, it means something, but for a common man, it may not make much sense.
MPFI means – Multi-point Fuel-Injection (also called fuel-injection system)
The term MPFI is used to specify a technology used in Gasoline/petrol Engines. For Diesel Engines, there is a similar technology called CRDI. We will discuss CRDI in a separate article to avoid confusion.
MPFI System is a system which uses a small computer (yes, a small computer without keyboard or mouse, its more like a microchip) to control the Car’s Engine. A Petrol car’s engine usually has three or more cylinders or fuel burning zones. So in case of an MPFI engine, there is one fuel –injector installed near each cylinder, that is why they call it Multi-point (more than one points) Fuel Injection.
In plain words, to burn petrol in an Engine to produce power, Petrol has to be mixed with some air, ignited in a cylinder (also called combustion chamber), which produces energy and runs the engine. I will not talk of further internal details because it will make this article for Engineers and not common people.
Before MPFI system was discovered, there was a technology called “Carburetor”. Carburetor was one chamber where petrol and air was mixed in a fixed ratio and then sent to cylinders to burn it to produce power. This system is purely a mechanical machine with little or no intelligence. It was not very efficient in burning petrol, it will burn more petrol than needed at times and will produce more pollution. But with the advancement of technology this was about to change.
MPFI emerged an Intelligent way to do what the Carburetor does. In MPFI system, each cylinder has one injector (which makes it multi-point). Each of these Injectors are controlled by one central car computer. This computer is a small micro-processor, which keeps telling each Injector about how much petrol and at what time it needs to inject near the cylinder so that only the required amount of petrol goes into the cylinder at the right moment.
So the working of MPFI is similar to Carburetor, but in an improved way, because now each cylinder is treated independently unlike Carburetor. But one major Key difference is that MPFI is an intelligent system and Carburetor is not. MPFI systems are controlled by a computer which does lots of calculations before deciding what amount of petrol will go into what cylinder at a particular point in time. It makes that decision based on the inputs it reads.
For the Inputs, the microprocessor (or car’s computer) reads a number of sensors. Through these sensors, the microprocessor knows the temperature of the Engine, the Speed of the Engine, it knows the load on the Engine, it knows how hard you have pressed the accelerator, it knows whether the Engine is idling at a traffic signal or it is actually running the car, it knows the air-pressure near the cylinders, it knows the amount of oxygen coming out of the exhaust pipe.
Based on all these inputs from the sensors, the computer in the MPFI system decides what amount of fuel to inject. Thus it makes it fuel efficient as it knows what amount of petrol should go in. To make things more interesting, the system also learns from the drivers driving habits. Modern car’s computers have memory, which will remember your driving style and will behave in a way so that you get the desired power output from engine based on your driving style. For example, if you have a habit of speedy pick-up, car’s computer will remember that and will give you more power at low engine speeds by putting extra petrol, so that you get a good pick-up. It will typically judge this by the amount of pressure you put on accelerator.
So the cars of today are really intelligent, well not as intelligent as drivers ;) but fairly intelligent to keep pollution under control and saving the fuel.

What is CRDI Engine ?

Fuel rail, injectors and fuel pressure regulat...
Fuel rail, injectors and fuel pressure regulator from a Honda 4 cylinder engine (D15A3). (Photo credit: Wikipedia)


    The common rail system prototype was developed in the 1960’s by Robert Huber of Switzerland and the technology further  developed by Dr.Marco Ganser at the swiss Federal Institute of Technology in Zurich.
                   The first successfull usage in production vehicle began in Japan by the mid-1990’s. Dr.Shohei Itoh & Masahina Miyaki of the Denso Corporation.

    ” Note: CRDI are governed by an Engine Control Unit(E.C.U) which opens 
    each injector electronically rather than mechanically.”

     The CRDI has brought a revolution in Diesel Engine 

                       OPERATING PRINCIPLE

    ->Here we use a Solenoid or Piezoelectric which makes fine electronic 
    control over fuel injection.
    ->The high pressure (CRDI) technology provides better fuel (atomisation) 
    i.e small droplets.
    ->Engine control unit (ECU) injects small amount of fuel this reduces its 
    expolsiveness & vibration.
    ->The fuel is supplies to injector by a common fuel rail which can be 
    maintained upto 2,000 bars (29,000).(The common rail supplies multiple 
    fuel Injector’s weight high pressure).
    ->The Injector’s are controlled by (E.C.U).When these fuel Injectors are
    electrically activated,a hydraulic valve(consisting of a nozzel and plunger)
    is mechanically or hydraulically opened & fuel is sprayed into cylinder at
    desired pressure.
    (Since: The Injectors are electrically operated,the Injections pressure at the
    start and end of Injections remains same as of fuel rail).
    (If the rail, pump & plumbing are sized properly.The Injections pressure 
    will be same in multiple Injections).
    ->It involves injection in stages such as PRE INJECTION,MAIN INJECTION 

    Why CRDI comes in Existence?

    In previous times the fuel Injection involves the basic two process

    1.Unit-Injection system.

    2.The distributor/inline pump system.

    Since these provide accurate fuel
    quantity & Injection time control’s they lay behind by some factors which
    are as follows.
    ->The following system were worked by the driven of cam.This means
    the highest Injection pressure can only be obtained at highest engine 
    speed & minimum Injection pressure at lowest engine speed.
    ->They were limited in number of Injection in single combustion process.


    ->Lower’s Engine noize.
    ->Lower’s vibration in Engine.
    ->Optimize quantity of fuel injection.
    ->It requires no heating times or less than 1sec.

What is the difference between 4 stroke and 2 stroke engines ?

This image was selected as a picture of the we...
This image was selected as a picture of the week on the Czech Wikipedia for th week, 2008. (Photo credit: Wikipedia)
Internal combustion Engines are classified in two categories: 2 Stroke and 4 stroke Engine. Many of us have confusion regarding the difference between 2 Stroke and 4 Stroke Engine. Below given table shows some of the key differences between two of them.
Sr.No 2 Stroke 4 Stroke
1. The thermodynamic cycle is completed in two strokes of the piston or in one revolution of the crankshaft. Thus one power stroke is obtained in each revolution of the crankshaft. The thermodynamic cycle is completed in four strokes of the piston or in two revolutions of the crankshaft. Thus, one power stroke is obtained in every two revolutions of the crankshaft.
2. Because of the above, turning moment is more uniform and hence a lighter flywheel can be used. Because of the above, turning moment is not so uniform and hence a heavier flywheel is needed.
3. Because of one power stroke for every revolution, power produced for same size of engine is twice, or for the same power the engine is lighter and more compact. Again, because of one power stroke for two revolutions, power produced for same size of engine Is less, or for the same power the engine is heavier and bulkier.
4. Because of one power stroke in one revolution greater cooling and lubrication requirements. Higher rate of wear and tear. Because of one power stroke in two revolutions lesser cooling and lubrication requirements. Lower rate of wear and tear.
5. Two-stroke engines have no valves but only ports (some two-stroke engines are fitted with conventional exhaust, valve or reed valve). Four-stroke engines have valves and valve actuating mechanisms for opening and closing of the intake and exhaust valves.
6. Because of light weight and simplicity due to the absence of valve actuating mechanism, initial cost of the engine is less. Because of comparatively higher weight and complicated valve mechanism, the initial cost of the engine is more.
7. Volumetric efficiency is low due to lesser time for induction. Volumetric efficiency is more due to more time for induction.
8. Thermal efficiency is lower; part load efficiency is poor. Thermal efficiency is higher: part load efficiency is better.
9. Used where low cost, compactness and light weight are important, viz., in mopeds, scooters, motorcycles, hand sprayers etc. Used where efficiency is important, viz., in cars, buses, trucks, tractors, industrial engines, Aeroplanes, power generation etc.

How Four Stroke Engine Works ?

Four-stroke cycle (or Otto cycle) 1. Intake 2....
Four-stroke cycle (or Otto cycle) 1. Intake 2. Compression 3. Power 4. Exhaust (Photo credit: Wikipedia)
four-stroke cycle engine is an internal combustion engine that utilizes four distinct piston strokes (intake, compression, power, and exhaust) to complete one operating cycle. The piston make two complete passes in the cylinder to complete one operating cycle. An operating cycle requires two revolutions (720°) of the crankshaft. The four-stroke cycle engine is the most common type of small engine. A four-stroke cycle engine completes five Strokes in one operating cycle, including intake, compression, ignition, power, and exhaust Strokes.
Intake Stroke
The intake event is when the air-fuel mixture is introduced to fill the combustion chamber. The intake event occurs when the piston moves from TDC to BDC and the intake valve is open. The movement of the piston toward BDC creates a low pressure in the cylinder. Ambient atmospheric pressure forces the air-fuel mixture through the open intake valve into the cylinder to fill the low pressure area created by the piston movement. The cylinder continues to fill slightly past BDC as the air-fuel mixture continues to flow by its own inertia while the piston begins to change direction. The intake valve remains open a few degrees of crankshaft rotation after BDC. Depending on engine design. The intake valve then closes and the air-fuel mixture is sealed inside the cylinder.
The compression stroke is when the trapped air-fuel mixture is compressed inside the cylinder. The combustion chamber is sealed to form the charge. The charge is the volume of compressed air-fuel mixture trapped inside the combustion chamber ready for ignition. Compressing the air-fuel mixture allows more energy to be released when the charge is ignited. Intake and exhaust valves must be closed to ensure that the cylinder is sealed to provide compression. Compression is the process of reducing or squeezing a charge from a large volume to a smaller volume in the combustion chamber. The flywheel helps to maintain the momentum necessary to compress the charge.
When the piston of an engine compresses the charge, an increase in compressive force supplied by work being done by the piston causes heat to be generated. The compression and heating of the air-fuel vapor in the charge results in an increase in charge temperature and an increase in fuel vaporization. The increase in charge temperature occurs uniformly throughout the combustion chamber to produce faster combustion (fuel oxidation) after ignition.
The increase in fuel vaporization occurs as small droplets of fuel become vaporized more completely from the heat generated. The increased droplet surface area exposed to the ignition flame allows more complete burning of the charge in the combustion chamber. Only gasoline vapor ignites. An increase in droplet surface area allows gasoline to release more vapor rather than remaining a liquid.
The more the charge vapor molecules are compressed, the more energy obtained from the combustion process. The energy needed to compress the charge is substantially less than the gain in force produced during the combustion process. For example, in a typical small engine, energy required to compress the charge is only one-fourth the amount of energy produced during combustion.
The compression ratio of an engine is a comparison of the volume of the combustion chamber with the piston at BDC to the volume of the combustion chamber with the piston at TDC. This area, combined with the design and style of combustion chamber, determines the compression ratio. Gasoline engines commonly have a compression ratio ranging from 6:1 – 10:1. The higher the compression ratio, the more fuel-efficient the engine. A higher compression ratio normally provides a substantial gain in combustion pressure or force on the piston. However, higher compression ratios increase operator effort required to start the engine. Some small engines feature a system to relieve pressure during the compression stroke to reduce operator effort required when starting the engine.
The ignition (combustion) event occurs when the charge is ignited and rapidly oxidized through a chemical reaction to release heat energyCombustion is the rapid, oxidizing chemical reaction in which a fuel chemically combines with oxygen in the atmosphere and releases energy in the form of heat.
Proper combustion involves a short but finite time to spread a flame throughout the combustion chamber. The spark at the spark plug initiates combustion at approximately 20° of crankshaft rotation before TDC (BTDC). The atmospheric oxygen and fuel vapor are consumed by a progressing flame front. A flame front is the boundary wall that separates the charge from the combustion by-products. The flame front progresses across the combustion chamber until the entire charge has burned.
Power Stroke
The power stroke is an engine operation Stroke in which hot expanding gases force the piston head away from the cylinder head. Piston force and subsequent motion are transferred through the connecting rod to apply torque to the crankshaft. The torque applied initiates crankshaft rotation. The amount of torque produced is determined by the pressure on the piston, the size of the piston, and the throw of the engine. During the power Stroke, both valves are closed.
Exhaust Stroke
The exhaust stroke occurs whenspent gases are expelled from the combustion chamber and released to the atmosphere. The exhaust stroke is the final stroke and occurs when the exhaust valve is open and the intake valve is closed. Piston movement evacuates exhaust gases to the atmosphere.
As the piston reaches BDC during the power stroke combustion is complete and the cylinder is filled with exhaust gases. The exhaust valve opens, and inertia of the flywheel and other moving parts push the piston back to TDC, forcing the exhaust gases out through the open exhaust valve. At the end of the exhaust stroke, the piston is at TDC and one operating cycle has been completed.