Aero Diesel Engines

Many of you may have heard recently that a significant milestone has been reached by one of the leading aviation Diesel engine designs. SMA, who have developed a four cylinder, air cooled diesel engine for light aircraft, has now gained FAA certification for their 230 HP SR305 engine. SMA are busy setting up retrofit centers around the world under the name of AeroNexx and currently have interest from Cessna, Socata, Maule and Sirrus amongst others.

The interesting thing about this engine, and many of the other aero diesel projects, is that it is designed to burn Jet fuel rather than normal road diesel. As a result I thought that this would be a good opportunity to talk about the diesel (or more correctly, compression ignition) cycle, and difficulties surrounding the decision to use Jet fuel to power these engines.

Jet fuel is classed as a kerosene fuel, which is somewhere between the gasoline and diesel fractions when distilling crude oil. This makes it a lighter fuel than diesel, but not as volatile as gasoline. The advantage over diesel fuel for the aviation application, other than it's availability on airfield, is that Jet fuel’s freeze point is much lower than for diesel fuel; -47 deg C compared to around 0 deg C for untreated diesel.

This means that the fuel is essentially unaffected by altitude exposure where the atmospheric temperature can be significantly below freezing. From a product control viewpoint, it is also much safer to use Jet fuel rather than road diesel as the requirements for aviation fuel means that it undergoes much more rigorous handling, control and testing than diesel fuel, so it's quality at the point of delivery is much more controlled.

The problems of using Jet fuel center on two main issues.

Firstly it has much less lubricity than diesel fuel. As Jet is a lighter fraction than diesel, it is much less "oily" and so, whereas road diesel engines commonly use the fuel to lubricate parts like the fuel pump, this is not a practical proposition when using Jet fuel. This is a problem that we commonly see when people have been running airfield bowsers on jet fuel - the engine will run, but soon the fuel pump will fail.

Secondly, and more significantly, the Cetane rating of Jet fuel can be much less than that of diesel. So what is a Cetane rating?

Diesel engines work by using a piston to compress air (without an fuel) to a very high degree (around twice as much as would happen in a gasoline engine). What this does is heat up the compressed air, in much the same way as a bicycle pump will become hot when it is being used - the heat comes from the compression of the air in the pump.

In a compression ignition engine, the heat of the air, when it is compressed, is high enough to cause a fuel to automatically ignite if it is injected into it, without the need to use a spark as is the case in a gasoline engine. Simplistically, this burning fuel then expands, causing the piston to be driven downwards, which creates power. The problem is controlling the burning so that the power is developed in the correct point of the engine cycle, and also so that very high pressures are avoided in the cylinder, which can cause damage.

What actually happens in a diesel engine is that when the fuel is injected there are 3 stages to the burning process.

Phase 1
When the fuel is first injected into the very hot air in the cylinder, it does not ignite immediately. The fuel is actually in the form of very fine droplets at this stage, and initially the drops start to evaporate to form a vapour. This vapour must undergo some chemical reactions and also mix with the surrounding air before any combustion can occur. This initial period is commonly known as the "Ignition Delay" period. As we shall see, a short delay period is preferred.

The delay period is primarily a property of the fuel being used, but can also be affected by the temperature of the cylinder contents, and also other factors such as whether the fuel is sprayed against a hot surface, or there is a lot of turbulence in the air / fuel mixture. It is a measurement of the delay period that is classified as the Cetane rating of the fuel - a high Cetane number results in a short delay period.

Phase 2
After the delay period, there is a period of rapid combustion of all the fuel which has had time to evaporate and mix with the surrounding air during the delay period.

The rate and extent of burning during this period are closely associated with the length of the delay period, as a long delay will result in all the majority of the fuel having evaporated and become mixed with air which means that there will be a rapid increase in pressure as all of the fuel burns in a short space of time. This is undesirable as the pressure rises can be very large and cause damage in the engine or, if less severe, can be heard as a noisy "knocking" sound in the engine. (See Chart 2)

One way to alter this is to use a fuel with a short delay period - or high Cetane rating. If the delay is short, then it is possible to still have fuel injection occurring whilst some of the fuel is starting to burn. So part of the fuel charge is in stage 2 (rapid combustion), whilst some fuel is still in phase 1 (delay). In this way the rapid combustion phase is spread out over a longer time and, as not all of the fuel charge is burning at once, the peak pressures are greatly reduced. (See Chart 1)

If we consider the things that effect delay time other than the fuel, then cylinder temperature tends to increase delay. This is why diesel engines tend to be much noisier when started from cold, than when they are running at operating temperature. With a cold cylinder, more of the fuel will evaporate before combustion starts, so there is more fuel burning at once, so the pressure increase is larger which is heard as a knocking sound.

Phase 3
The Third phase of combustion is the period from maximum pressure to the point where combustion is measurably complete. If the delay time is longer than the injection time (which we have seen results in large pressure increases in Phase 2) then there is comparatively little fuel remaining to burn in phase 3. Phase 3 is characterised by a decreasing pressure in the cylinder and only the fuel which has not found enough oxygen to react with in phase 2 remains to be burned in this period.

You can see from the accompanying charts the effect of a long delay period. This is exactly the situation that must be accounted for when deciding to use Jet fuel in a diesel engine as the Cetane rating of Jet fuel can be around half that of diesel fuel. If the engine is to be designed to run on Jet fuel, then the designer must be careful to allow for this.

There are several different techniques that can be used to try to reduce the delay period of the fuel such as deliberately injecting onto a hot surface such as the piston crown, encouraging the air to swirl rapidly to encourage mixing of the air and fuel, designing the engine so that the outer edge of the piston gets very close to the cylinder head so that the air and fuel are pushed into a central indentation in the piston crown. This last technique will make the air swirl more rapidly in the center of the piston in the same way that a dancer will spin more rapidly when he draws his arms in. Often diesel engines will also have a "glow plug" in the top of the cylinder to aid starting. This acts as a hot surface and helps to both cause the fuel to evaporate and also reduce the delay period when the engine internals are cold.

One significant advance, and one used on the SMA engine, is the computer control of the fuel injection to optimise performance. This is commonly called the "single lever" concept, as there is only one lever in the cockpit, with a computer controlling fuelling, engine and propeller speed, power setting, injection timing etc.

Shell is proud to have been a partner in the development of the SMA engine for some years now. It has not only given us the opportunity to help on fuel issues, but it has given us valuable experience in the development and testing diesel engine oils suitable for this new generation of diesel engines. It is not only the engine design that is effected by the use of Jet fuel, but there is also some significant implications for the lubricant used, and this is something that Shell have been working on for some years now.

With the FAA certification of the SMA engine I am sure that we are about to see some significant new horizons opening up in the General Aviation market.