Things Most People Don’t Know About Combustion in CI Engine


Combustion in CI engine is very different from combustion in SI engine.  

Whereas combustion in an SI engine is essentially a flame front moving through a homogeneous mixture, combustion in CI engine is an unsteady process occurring simultaneously at many spots in a very non-homogeneous mixture at a rate controlled by fuel injection

The air intake to the engine has not been adjusted and engine torque and power are controlled by the amount of fuel injected per cycle.  

Since the inflow air is not throttled, the pressure inside the intake manifold is constant at a value close to 1 atm. 

As a result, the engine cycle is very small and therefore has better thermal efficiency compared to SI engines.  

This is especially true at low speeds and light loads when a SI engine with a lot of pump work is running at half throttle.  

For the CI engine,  

Wnet = Wgross – Wpump = Wgross  

During the compression stroke, only air is present in the cylinder. 

Latest CI compression ratio range is 12 to 24.  

Higher thermal efficiency (fuel conversion efficiency) is achieved at these compression ratios compared to a normal SI engine.  

The CI engine operates very slim (equivalent ratio cp = 0.8) and the braking force is reduced. 

Fuel is injected into the cylinders late in the compression stroke by one or more injectors located in each cylinder combustion chamber.  

Injection time is usually about 20° of crankshaft rotation, starting at about 15° before top dead center and ending about 5° after top dead center. 

The ignition delay is fairly constant in real time, that is, at higher engine speeds.  

Fuel injection should start a little earlier in the cycle.  

In addition to swirl and turbulence in the air, there is a high injection speed had to be distributed throughout the cylinder and mixed with the air. 

After injection, the fuel must go through a series of events to ensure a proper combustion process:  

1. Atomization.  

Fuel droplets split into very small droplets.  

The smaller original droplet size of the injector, the faster and more efficient for spraying process.  

2. Vaporization.  

Small droplets of liquid fuel evaporate into vapor.  

It happens very quickly due to the hot air temperature produced by high compression of the CI engine.  

This evaporation process requires high temperatures. 

The minimum compression ratio of the CI engine should be about 12:1.  

About 90% gets vaporized within 0.001 seconds of fuel injected into the cylinder after injection. 

When the first fuel evaporates, the area in the immediate surroundings is cooled by evaporative cooling.  

This has a strong effect on subsequent evaporation.   

Near the core of the fuel jet, with high fuel concentration evaporative cooling leads to adiabatic saturation of the fuel.  

Evaporation stops in this area and occurs only after additional mixing and heating while this fuel evaporates. 

3. Mixing 

After vaporization, the fuel vapor must be mixed with air to form an air-fuel mixture in the flammable area.  

This mixture with high fuel injection velocities added to the swirl and turbulence in the cylinder show a non-uniform distribution of the air-fuel ratio.  

It occurs around the injected fuel jet.  

Combustion limits of equivalent ratios of cP = 1.8 (fat) and cP = 0.8 (fat).  

4. Self ignition.  

Approximately 8° before top dead center, 6-8° after the start of injection, the air-fuel ratio is the mixture which begins to ignite on its own. 

Prior to actual combustion, secondary reactions occur, such as the decomposition of large hydrocarbon molecules into smaller hydrocarbon molecules species and some oxidation.  

These reactions are caused by high temperatures. 

The air is heat-generating and quickly raises the temperature in the local proximity.  

This ultimately leads to the actual ongoing combustion process.  

Fuel jet of a CI engine showing air-fuel vapor zones around the inner liquid core. The liquid core is surrounded by successive zones of vapor which are (A) too rich to burn, (B) rich combustible, (C) stoichiometric, (D) lean combustible, and (E) too lean to burn. Self-ignition starts mainly in zone B. Solid carbon soot is generated mostly in zones A and B.

5. Combustion.  

For many, combustion begins at the same time as self-ignition, slightly rich zone of fuel jet with equivalent ratio = 1 to 1.5.  

At this point, from 70% to 95% of the fuel in the combustion chamber is in a vapor state.  

When combustion begins and multiple flame planes spread from multiple automatic ignitions site quickly consumes all gas mixtures in the correct combustible state air-fuel ratio even if self-ignition does not occur.  

This gives something very fast increased temperature and pressure in the cylinder.  

Higher temperatures and pressures reduce evaporation time and ignition delays the time of extra fuel particles and causes more self-ignition points to further increase the combustion process.  

Liquid fuel is still being injected into the cylinder after the first fuel is already burning.  

The combustion rate, which has come to be controlled by the injection rate. 

A slower pressure rise that occurs after the initial fast rise.  

Burning continues with an engine speed of about 40° to 50°, it is much longer than a 20° fuel injection.  

This is because it takes a long time for some fuel particles to mix with air to form a flammable mixture, so called combustion power stroke.  

This can be seen where the pressure remains high until the piston reaches 30° to 40° after top dead center.  

About 60% of fuel burns first-third of the burning time. 

Combustion speed increases with the engine speed so the angle remains almost constant.  

Between the main parts of combustion, 10% to 35% of the fuel vapor in the cylinder will be in a combustible AF. 


As soon as the air-fuel mixture reaches the combustible air-fuel ratio and temperature is hot enough to self-ignite, ignition delay is still in the range of 0.4-3 msec (0.0004 to 0.003 seconds).  

As increase in temperature, pressure, engine speed and or rise in compression ratio reduces the ignition delay time.  
Fuel droplet size, Fuel speed, injection rate, and physical characteristics of the fuel seem to have little or no effect on delay time.  

The wall will heat up and the ignition delay will decrease in real time.  

But since the ID is almost constant over the cycle time, the crankshaft angle is fairly constant.  

If the injection is too fast, the temperature will rise and the ignition delay time will be long.  

The pressure will be lower, if the injection is delayed, the piston will pass through the TDC (top dead center), the pressure and temperature will drop, and the ignition delay time will be extended again.  

 It is important to use a cetane fuel suitable for a particular engine.  

Cetane number is a measure of ignition delay and should be adapted to a particular engine cycle and injection method.  

If the cetane number is low, the ignition delay is too long and more fuel than desired is injected into the cylinder before combustion begins.  

Then, when combustion begins, more fuel will be generated. 

A very large initial force is generated on the piston surface, which makes the engine cycle rough.  

If the cetane number is high, combustion starts too early before TDC, as a result, engine power is lost.  

The usual cetane numbers for the most common fuels are in the range of 4060.  

In this range, the ignition delay time is inversely proportional to the cetane number.  

ID α l / CN 

There is also a strong inverse correlation between the cetane number and the octane number.  

CN α liON  

Cetane number can be changed by blending a small number of certain additives to fuel.  

Inflammation-promoting additives include nitrites, organic peroxides, and some sulfur compounds.  

Due to its high-octane number, alcohol is not sufficient as a fuel for CI engines.  


The flame of the CI engine is highly non-uniform.  

A flame is generated when spontaneously ignited quickly swallow all parts of the air-fueled combustion chamber at the rate of flammability.  

AF is slightly flammable and very large in very fuel-rich areas. 

A large amount of solid carbon particles is produced. 

As the combustion process progresses and the air-fuel mixture in the combustion chamber continues to be mixed by the swirl and turbulence, most carbon particles react further, only very small one percentage eventually ends up in the environment. 

Solid carbon particles are fuels that react with oxygen when the correct mixture is obtained.  

C(s) + 02 ~ CO2 + heat  

In that the overall air-fuel ratio is lean in a CI engine, most of the carbon will find and react with the excess oxygen even after the air-fuel mixture leaves combustion chamber, additional reaction takes place in the exhaust system of amount of solid carbon.  

In addition, most CI engine exhaust systems have a particulate filter that filters much of the remaining solid carbon.  

Only a small portion of the original solid carbon particles formed in the combustion chamber are released into the environment.  

The CI engine operates to keep the exhaust smoke (soot) within acceptable limits.   

The sum of these motors in stoichiometric AF fibrillation, the amount of exhaust smoke is unacceptable.   

Even with the lean manufacturing system, many metropolitan areas are very worried about diesel emissions from trucks and buses. 

Very strict laws are imposed in many places reducing emissions requires bus and truck operations and significant improvements emissions from these vehicles.  
The CI engine operates on unthrottled intake and controls engine output and the amount of fuel injected.  


The mechanical efficiency of the CI engine is high.  

The CI engine has higher thermodynamic efficiency.  
The CI engine provides higher fuel conversion efficiency.  

The CI engine has higher volumetric efficiency.  


These types of fuels that can be utilized are limited to very high quality gaseous and liquid fuels.  
As like gasoline and diesel this fuel used is very expensive. 
The emissions of an engine are basically higher than those of an internal combustion engine.  
Not comfortable for large-scale power generation. 

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