Tuesday, 22 October 2013

Engine Combustion - Spark Ignition (Gasoline)

Engine combustion is a fascinating topic to gain an understanding of, particularly when comparing compression and spark ignition, the fuels used and their properties needed for each respective type. Looking at details, and the current trends in technology. It’s not hard to see the convergence and similarity between gasoline and diesel engine fuel systems and combustion. Let's look in detail first at spark ignition based combustion

Spark ignition
For spark ignition combustion, the mixture is prepared completely prior to combustion (outside of the combustion chamber) - that is, the fuel is introduced to the air - fully atomised - and in theory, this mixture is uniform in its distribution. So called ‘homogeneous’, the amount of fuel in proportion to air should be chemically correct. What this means is that there is enough air, containing oxygen, to fully oxidise or ‘burn’ all of the fuels volatile content. Sounds quite straight forward, but in practice, not that easy considering all the operating conditions that a vehicle engine has to encounter. This mixture is compressed in the cylinder as the cylinder volume decreases due to the piston rising towards top dead centre (TDC), the pressure increases, with reference to simple gas laws, the temperature of this mixture also increases, but not sufficiently to reach the ignition point of the fuel/air mixture.

So far then we have mixed fuel and air and compressed it. But there are many hurdles for the engine designer to overcome to be able to do this efficiently for a multi-cylinder engine. These days, fuel is injected into the air stream, near the inlet port, but remember the carburettor (or even a single point injector). The mixture preparation occurs away from the point of entry into the cylinder, thus, distributing the mixture evenly to each cylinder, with the same amount of fuel for each cylinder, for a given operating condition, was a real headache for the engine designer.

Fig 1 - Single a) and Multi-point b) injection system layout 

Why? - well, in order to get the best possible performance and efficiency out of an engine, the individual cylinder contributions must be as even as possible, with as little variation as possible. Even small variations have a dramatic effect on the overall engine performance, so even mixture distribution is key to this, and impossible to achieve fully with a centralised mixture preparation system like a single carburettor shared between cylinders.. In addition, the distance that the mixture travels in order to get to the cylinder has another effect, that is the possibility that the fuel and air may separate during transit - the fuel literally drops out of the moving air becoming liquid droplets again (instead of a finely atomised spray). This is known as wall wetting, and causes flat spots due to instantaneous weak mixtures being introduced to the cylinder, this effect is much worse at low temperature (hence the need for the choke in days gone by, to enrich the mixture when cold) and during transient operation, where the air accelerates faster than the fuel (hence the need for an accelerator pump, to richen the mixture during accelerations). These were some of the arguments for the move to port fuel injection to each cylinder, thus contributing to improving efficiency and reducing emissions.

Back to combustion - fuel and air is mixed and compressed, now we are ready to produce some work. In a gasoline engine, I am  sure we all know that an electrical spark or arc is used to start combustion. We mentioned before that the mixture temperature is raised, but not beyond its ignition point. The intense electrical arc produced by the spark plug at it’s electrodes creates a localised heating of the mixture, sufficient for the fuel elements to begin oxidising and combustion of the mixture starts with a concentric flame front growing outwards from the initial ignition kernel. Once this process is initiated, it perpetuates itself, there is more or less no control over it. We just have to hope that the mixture is prepared correctly to sustain this flame so that is consumes all of the mixture, burning it cleanly and completely. The technical term for this type of combustion is ‘pre-mixed’. The engine is ‘throttled’ to control the mass of mixture in the cylinder, and hence its power output. The throttle being a characteristic of the gasoline engine.

Fig 2 - Flame propagation in a gasoline engine via optical imaging system (source: AVL)

An important point to note is that the speed that this flame travels across the combustion chamber is important, the typical  flame speed, travelling through an air/fuel mixture, would be far too slow in a combustion engine (approximately 0.5 metres per second). So, we have to speed things up. The way this is done is via cylinder charge motion, or turbulence. The turbulence is generated via induction and compression processes in conjunction with the combustion chamber design and has the effect a breaking up the flame front, increasing its surface area, thus increasing the surface area of fuel mixture for oxidation. Assuming a normal combustion event, the flame front grows out to the periphery of the cylinder where it decays once all the mixture is burned.

Fig 3 - Charge motion speeds up the combustion process, in a gasoline engine this is generally known as 'tumble'

Of course the timing of this event is essential! Ideally, we want the cylinder pressure, forcing down on the piston, to occur at the correct time relative to the crank angle. Seems obvious - too soon and we may be trying to push against the rising piston, too late and the piston is already moving down the bore, hence the expansion of the gas won’t do any work and the energy will be wasted as excessive heat in the exhaust.. A simple analogy would be to imagine pushing someone on a swing - too soon and the effect is collision, too late and the effect is no force transmitted - well, it’s the same in the engine cylinder. What engine engineers do know, is that half of the total fuel energy should be released at around 8 to 10 degrees after top dead centre. This can be measured by cylinder pressure analysis during an engine test. Hence, with a new engine design, the appropriate ignition timing can be mapped via monitoring the cylinder pressure for energy release, as well as knock, in order to map the correct value for a given engine operating condition. In summary then, the key points to consider regarding the spark ignition engine:
  • The fuel/air mixture is prepared externally, and ignited via a timed spark
  • The engine power is controlled via throttling, which reduces efficiency, particularly at part-load
  • The compression ratio is limited by self-ignition of the fuel/air mixture
  • In operation, engine maximum torque is limited by abnormal combustion (knocking)
  • Cylinder to cylinder variation (due to fuel distribution problems, and other factors) reduces the efficiency of the engine and is significant in a spark ignition engine

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