Spark Ignition and Compression Ignition Engines

Spark Ignition and Compression Ignition Engines

Spark Ignition Engines

 Internal combusiton engines are divided into spark ignition engines and compression ignition engines. Almost all automobiles today use spark ignition engines while trailers and some big trucks use compression ignition engines. The main di erence between the two is the way in which the air to fuel mixture is ignited, and the design of the chamber which leads to certain power and e ciency characteristics. Spark ignition engines use an air to fuel mixture that is compressed at high pressures. At this high pressure the mixture has to be near stoichiometric to be chemically inert and able to ignite. Stoichiometric means that there is a one to one ratio between the air and fuel mixture. So the mixture in order to ignite needs not to be either with too much fuel or too much air but rather have an overall even amount. There are several components to the spark ignition engine. Chamber design, mixture and the injection system are some of the most important aspects of the spark ignition engine. The importance of the chamber design will be discussed. The four basic designs for combustion chambers are as follow:


the distance travelled by the ame front should be minimised


the exhaust valve and spark plug should be close together  


there should be su cient turbulence


the end gas should be in a cool part of the combustion chamber.

The rst design requires that the distance between the end gas and the spark plug be close in order for combustion to progress rapidly. If combustion is sped up then, (i) the engine speed is increased and therefore power output is higher, and (ii) the chain reactions that lead to knock are reduced. From the second design criteria the exhaust valve, since it is very hot, should be as far from the end gas in order to prevent knock or pre-ignition. The third design criteria suggest that there should be enough turbulance in order to \promote rapid combustion", through mixing. (Stone, p.126) Too much turbulance, however, will lead to excessive heat transfer from the chamber and too rapid combustion which causes noise. Turbulance in combustion chambers is generated by squish areas or shrouded inlet valves. The fourth design requires that the end gas be in a cool part of the combustion chamber. The cool part of the combustion chamber forms between the cylinder head and piston. There are many types of designs for combustion chambers. Four common combustion chambers are

wedge chamber

 hemispherical head

 bowl in piston chamber

bath-tub head

The wedge design is simple giving good results. In the wedge design the \valve drive train is easy to install, but the inlet and exhaust manifold have to be on the same side of the cylinder head." (Stone, p.127) The second type of combustion chamber is the hemispherical head. The advantage of a hemispherical chamber is its angled valves which are used in high performance engines. This design is expensive with twin overhead camshafts. The design allows for cross ow from inlet to exhaust, with cross ow occuring at the end of the exhaust stroke and at the beginning of the induction stroke while both valves are open. The third combustion chamber is a cheaper design that has good performance. The last combustion chamber design has a \compact combustion chamber that might be expected to give economical performance."

The process by which the air to fuel mixture is prepared and put in the combustion chamber is through carburetors and fuel injectors. Spark plugs are part of all spark ignition engines. In order to start one of these engines a spark has to ignite a mixture into a ame. The way in which this spark is rst initiated is through the car battery and a circuit directly leading to the spark plug. The battery supplies the electic current to initiate a spark in the spark plug. The Spark then ignites the air and fuel mixture. The type of fuel injectors used divide into multi-point and single-point injection. Carburettors divide into xed and variable jet carburettors. The air and fuel mixture is analysed as either a lean or rich mixture depending on the content of fuel. A stoichiometric mixture is one in which there is a perfect ratio of air and fual molecules. A lean mixture would be de cient in fuel where a rich one would be saturated with fuel. To achieve economic status and yet receive the maximum power the engine would have to use a lean mixture and a rich one at full throttle. When the throttle is fully opened and a lean mixture is used the power output is economical because of the weak fuel. When the throttle is opened the combustion chamber needs the air to fuel mixture. Since a stream of air is generated extra fuel is needed to compensate for the insu cient ow of fuel. In order to obtain maximum power a rich mixture is needed. For good fuel economy all the fuel should be burnt and the \quench area where the ame is extinguished should be minimised."

Compression Ignition Engines

Compression ignition engines di er from spark ignition engines in a variety of ways but the most obvious one being the way in which the air and fuel mixture is ignited. As stated above a spark plug is used to create a spark in the combustion chamber which ignites the mixture. In a compression ignition engine there is no spark to create the ame but rather high temperatures and pressures in the combustion chamber cause a ame to initiate at di erent sites of the combustion chamber. Combustion increases with increasing pressure and temperature. Compression ignition engines are divided into direct and indirect ignition engines. Diesel engines require fuel injection systems to inject fuel into the combustion chamber. Fuel injection systems are either linear or rotary. Rotary fuel injectors are used in indirect ignition engines because of low pressures. 

Direct injection engines use pressures of up to 1000 bars to inject fuel into the combustion chamber. High pressure is needed because the heat addition process takes place at a compressed state, so in order for the fuel to inject well the pressure has to be greater than the one that has been accumulated through compression. There are several engineered direct injection combustion chambers. This goes to show that the actual design of compression ignition engines is not as critical as the design considered for spark ignition engines. Swirl is the most important air motion in the Diesel engine. The importance of swirl is that it mixes the air and fuel so that combustion can increase. The direction of swirl is at a downward angle so that proper mixing can take place. The compression ratio for direct ignition engines is usually between 12 : 1 and 16 : 1.

Indirect ignition engines have a pre-combustion chamber where the air to fuel mixture is rst stored. The purpose of the separate chamber is to speed up the combustion process in order to increase the engine output by increasing the engine speed. The two basic combustion systems are the swirl and pre-combustion chambers. Pre-combustion chambers depend on turbulance to increase the combustion speed and swirl chambers depend on the uid motion to raise combustion speed. In divided chambers the pressure required is not as high as the pressure required for direct ignition engines. The pressure required for both type of divided chambers is only about 300 bars.

With all Diesel engines there is some type of aid to help combustion. Electrical components aid in the initiation of the combustion process by using an electrical source, such as a car battery, to heat themselves and transfer the energy to the mixture for combustion. Cold starting a Diesel engine is very di cult without the use of these tabs that conduct an electric current. When electrical elements heat up and the air to fuel mixture comes in close contact with the tab then a combustion occurs. The Diesel engine has high thermal e ciencies, and therefore low fuel consumption. The disadvantage of Diesel engines is their low power output, relative to their weight, as compared with spark ignition engines.