Why do Diesel engines have a higher compression ratio than gasoline engines?

Question

Are Diesel engines constructed differently, so that they have a higher stroke volume? (longer stroke, smaller combustion chamber etc.) I know the main differences between Diesel and Gasoline engines(fuel injection - spark plug etc.), but i can't get it in my head why Diesel engines have a higher compression ratio. Is it because they are designed different to have a greater volume?

Let's ask my question in a different way: What do you need to change of the parts in the engine to create a certain compression ratio? Which constructional factors do have influence on the compression ratio?

Answer 1

Are Diesel engines constructed differently, so that they have a higher stroke volume? (longer stroke, smaller combustion chamber etc.) Is it because they are designed different to have a greater volume?

Yes.

Let's ask my question in a different way: What do you need to change of the parts in the engine to create a certain compression ratio? Which constructional factors do have influence on the compression ratio?

As the name implies, the compression ratio is a ratio between volumes of the chamber between bottom dead centre and top dead centre. Increasing the ratio requires changes that are quite fundamental: longer stroke and/or smaller combustion chamber. Another caveat is that in diesels forces exerted on the piston, its rod, crankshaft and shaft bearings are higher so those components are heavier and sturdier. For this reasons, diesels and gasoline engines are designed separately, share no basic components and it's pretty much impossible to convert one to another.

The most common method of increasing compression ratio on existing engine is to skim the head - that is "lower the ceiling" in combustion chamber. This unfortunately makes everything mounted in the head (spark plug and possibly valves) closer to piston when it's in top position, so you have very limited room for skimming before piston starts touching things it's not supposed to touch.

Another problem is that shapes of combustion chambers in modern engines are very carefully designed to achieve good flame propagation and even combustion. So while in theory one could design a simple chamber insert just to occupy some volume and therefore increase the compression ratio, it would totally mess up the combustion (not even mentioning the possibility of it getting lose and wrecking the engine).

After all those "why it's impossible" reasons, here's one thing that can and actually is done: Because compression starts only after all the valves are closed, you can lower the compression by delaying closure of the intake valve. So, in first part of the "compression" stroke, air is expelled back into intake manifold (so no compression actually takes place), then the valve closes and only the remaining part of the stroke actually compresses. This trick is used in so called "Atkinson cycle" engines that achieve better fuel efficiency at cost of lower power, eg in Prius. (The extra efficiency comes from power stroke being longer than compression stroke, not from lowering compression ratio.)

Bottom line: by changing intake valve timing you could lower the compression ratio almost all the way down to 1:1.

Answer 2

A very high compression leads to self-ignition of the fuel-air mix. In the case of the Diesel engine this self-ignition is desired and part of the concept. In the case of the Gasoline engine self-ignition is not part of the design and therefor harmful.

Some methods to influence the compression ratio.

• varying the width of the head-gasket
• milling piston cavities
• grinding out the combustion cavity in the head

A limiting factor for the length/diameter ratio of the cylinder is that the connecting rod touches the cylinder wall when the stroke is too long. This can be prevented when using a crosshead.

Answer 3

Its the high compression that ignites the diesel fuel, since it does not have spark plugs.

Answer 4

You asked

Why do Diesel engines have a higher compression ratio than gasoline engines?

Answer

Because compression is used to ignite the air fuel mixture and is described as homogeneous charge compression ignition. Diesel engines do not use spark plugs and must use compression in order to initiate the combustion process.

When the air and fuel charge is compressed by the piston on it's upstroke. Heat within the cylinder is compressed into a smaller area and the temperature rises due to the heat being compressed into that smaller area. Heat from compression is described well here.

Once the temperature of the compressed air fuel ratio reaches the point of ignition Homogeneous Charge Compression Ignition occurs. This point of ignition is when the air fuel charge ignites and the release of energy drives the piston back down, this is the unique attribute of the diesel engine.

Answer 5

What should be realized here is that diesel and gasoline are two different fuels with different physical and chemical properties.

It is easier to ignite gasoline than diesel. Light a match and throw it in a puddle of gasoline and it will happily burn at room temperature and pressure. The same isn't true of diesel; it will refuse to ignite. Video evidence here.

Because diesel doesn't ignite as readily as gasoline, it requires a higher pressure to self-ignite (assuming both fuels are at the same temperature).

A typical compression ratio for diesel engines is 18-22. Their gasoline counterparts will typically have their CR's lie in the 9-13 interval.

Answer 6

Per high school physics & chemistry, compressing a gas raises its temperature: if you compress it enough fast enough, the temperature will rise enough to ignite SOME gasses. Now you're asking yourself "But diesel is a liquid, not a gas, so what's this guy talking about?" The injectors on diesel engines are powerful, and inject the diesel fuel into the cylinder with such force that it vaporizes, and together with the air into which it's injected, becomes for all intents and purposes a gas. In fact, if you held a diesel injector while it ran, the fuel spray would tear your skin apart. Cold weather inhibits diesel's combustion, which is why diesel engines have "glow plugs": they warm up the cylinders, to counteract the cold temperatures at start. They turn on automatically: an icon on the gauge panel lights up to indicate that they're being used, then goes off when the engine can be started. There was a VW commercial in the 70's that showed a guy in a lab coat under the hood of a Rabbit, tossing out the distributor cap, rotor, spark plugs, plug wires, etc. A gallon of diesel has more potential energy than a gallon of gasoline.