Why Diesels are Different

SI Engines

All SI engines have air/fuel mixtures existing in the cylinder before combustion is initiated by the spark plug. These mixtures may exist continuously in the inlet manifold (in the case of carburettor or 'throttle body injection' systems). Or, they may exist only after each cylinder begins its induction stroke (in the case of 'traditional' individual inlet tract injection). Or they may exist for only a short period in each cylinder before the spark plug fires (in the case of Direct Injection).

If such an air/fuel mixture exists, it follows that the quantities of air and fuel must exist in proportions which can be expressed as a ratio - the air/fuel ratio. The stoichiometric ratio for air and petrol (gasoline) is roughly 15:1. That is, for complete combustion without excess air, we need 15 times the quantity of air as the quantity of fuel. If there is less air in relation to fuel (say 13:1 ratio), it is called a 'rich' mixture. If there is more air in relation to fuel (say 17:1 ratio), it is called a 'lean' mixture. SI engines will only operate over a fairly narrow range of air/fuel ratios - if it is hugely rich or lean, it won't ignite at all. Therefore, even at idle, a close to stoichiometric ratio must be maintained and, to keep the engine speed at 'idle' rpm, the mass of both air and fuel ingested is severely restricted by the throttle. In fact, the main throttle valve is just about fully closed and a relatively tiny bypass passage allows a small quantity of air (or air/fuel mixture, the case of carburetted engines) into the engine.

In SI engines, close to stoichiometric combustion is desirable under steady cruising conditions, to give optimum economy, and most engine management systems constantly sample the exhaust gas stream to keep the quantity of excess air (oxygen) at a very low but measurable level. However, during acceleration, better performance is obtained with slightly rich mixtures and the EMS will 'enrich' the mixture under these conditions. What must be avoided at all costs, however, is excessively lean mixtures, especially under heavy load conditions. The lean mixtures cause far higher peak combustion temperatures and lead to rapid erosion of the exhaust valve and valve seat and, in extreme conditions, can cause a hole to be burnt in the piston crown.

CI Engines

As combustion begins only when fuel injection begins in a CI engine, the notion of an air/fuel mixture ceases to have much meaning. A 'mixture', if it exists at all, exists only for the instant between the beginning of injection and the beginning of combustion - a matter of a few milliseconds at most. Similarly, air/fuel ratios have little meaning in CI engines. The air/fuel ratio does not exist as a semi continuous parameter which changes only over a narrow range, as is the case in SI engines.

Certainly there is an air/fuel ratio for each power stroke of each cylinder but it really only has some quantifiable value at the end of the fuel injection pulse, when all the fuel that is going to be injected in that power stroke has been delivered. And it is always (or should be) a very 'lean' ratio. CI engines operate most of the time with a large proportion of excess air.

Why is this so? Well, remember that CI engines only work if the cylinder contains a charge of air which has been compressed enough to heat it above the ignition temperature of the fuel. To ensure this, generally the inlet of air into a CI engine is un-throttled and roughly the same mass of air is ingested by each cylinder whether the engine is idling or at full load. [OK, I admit this is not strictly true in the case of forced-induction (turbo-charged or super-charged) CI engines, but that's a matter for more detailed discussion at a future date.]

What changes over a wide range is the quantity of fuel injected into each cylinder. At idle, only a tiny squirt of fuel is required to keep the engine 'ticking over'. So, the air/fuel ratio is extremely 'lean' compared to stoichiometric - perhaps more than 100:1 At steady low speed cruising, say between 60 to 80 km/h, it may be of the order of 40 or 50:1. At full load and full throttle, it may be approaching 20:1

So, why don't these very 'lean' ratios cause damage to CI engines, such as may occur in SI engines under 'lean' conditions? Well, consider this: extreme combustion temperatures occur when the air/fuel ratio in a cylinder is just on the lean side of stoichiometric. Once considerable excess air is available in the cylinder, this mass of un-reacted air is able to absorb much of the heat generated by combustion and the overall cylinder gas temperature is kept to relatively low levels. This energy is not wasted; the excess air expands as it absorbs heat and it all helps to push on the pistons.