Diesel peak cylinder pressure (and crank angle)

[murpia]

I have a simple air-standard model of the diesel cycle. The assumption is therefore heat release at constant pressure. This means my calculated peak cylinder pressure is dictated entirely by the compression ratio.

Obviously this is somewhat inaccurate. Can anyone help with an idea of the likely increase in cylinder pressure during combustion for a real engine, and the likely location (in crank angle) of this peak? Maybe a pressure ratio rule-of-thumb is applicable (e.g. 120% of the air-standard value)?

Many thanks, Ian

 

[EHudson]

A rough guess for non turbo diesel peak pressure is in the 1100 to 1300 psi range near TDC. Perhaps if you have an IMEP in mind someone can give you a better estimate.

The real pressure rise, over compression pressure, begins when fuel begins to burn, just a bit after injection.

Advancing the injection timing beyond some optimum will raise the cylinder pressure but power output will not increase much because the negative work required to bring the piston up to TDC offsets the gain on the down stroke. (You just put more strain on the components of the engine).

The speed at which the fuel burns (cetane rating) and the speed with which it can be injected are critical to good performance. (These factors actually limit the top speed of a diesel engine).

The electronic control of injectors and emissions requirements have changed the game in the last decade. (Later injection and lower peak pressures help to reduce NOx). Some strategies squirt a small charge into the combustion chamber before TDC to get an ignition source going, and then dump the main charge in later. I have heard (but not seen) that some have tried three different injection points in a cycle. I have only witnessed dual charge cycles. (This can lead to two pressure peaks in some cases)!

Such custom injection timings were not possible with the old pump-line-nozzle systems.

For an analysis of several types of diesels you might start with the March 1943 Journal of Applied Mechanics for Fredrick Porter’s article on the Harmonic Coefficients of Engine Torque Curves. His curves do not extend beyond 150 psi MEP for 4 cycle diesels but you can get a good idea of what the curve for a higher pressure will look like.

You could ask an injector manufacturer (Bosch, Siemans ?) for a typical pressure curve.

 

[SomptingGuy]

"I have heard (but not seen) that some have tried three different injection points in a cycle"

Some of today's HSDI diesels inject as many as 5 times per combustion event.

 

[turbomotor]

Hello Ian,

I've also searched for data to answer a question similar to yours. (I use a relatively crude, Excel based home-made model for evaluating Otto and Diesel air standard cycles.)

The only text that I've found that has reasonably modern and descriptive cylinder pressure Vs. volume data is Heywood's text. I have generated similar data myself with dyno test engines (using high speed piezo transducers to measure cylinder pressure).

There is no easy answer. Many modern diesel engines could be more accurately labeled "compression ignition Otto cycle engines". But with the increased use of high pressure common rail fuel injection systems and electronic rate shaping, combined with the requirement for lower Nox, the heat release cycle varies quite a bit comparing engines built ten years ago to engines built today.

Dick