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What’s so interesting about diesel?

Diesel Tuning Fundamentals

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Discussion and questions related to the course Diesel Tuning Fundamentals

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What’s so interesting about diesel? Or what makes it difficult?

To start a diesel engine dos not have an active ignition source like a spark(plug) used on a petrol engine for example. So when and how dos the combustion start????

What makes it combust? Well temperature will start the combustion on a diesel engine but before and even during combustion a load of things will take place.

What about diesel fuel? I like to split them in two parts. Low weight and heavy weight fuel fractions. Low weight fuel fractions is kind of evaporating in the same way as petrol dos so it need less temperature/energy to evaporate. Heavy weight fuel fractions? Some of it is Paraffin, the stuff you make candles from so try to melt this down very fast? It will take a load of temp and energy so during pre combustion or start of combustion a lot of temperature/volume expansion in the combustion chamber is taken out by evaporating the heavy fuel fractions so it will take some time before we get things up to speed/temperature burning the diesel a lot faster getting greater expansion in the combustion chamber.

So a diesel is all about a combination of start of injection timing, end of compression temperature (mainly related compression ratio and boost pressure in most cases) fuel pressure, injector nozzle hole size or diesel droplet size.

This all makes a load of variables that can be used and even the most simple diesel ECU will be over 5D tuning as for RPM/MAP or MAF/fuel pressure/injection timing/fuel metering/torque control. As for petrol it’s only 3D as for RPM/fuel metering/ignition timing to get things running in the most basic form.

If you get the picture diesel combustion is not a big bang and if you get things done correct you will spike combustion pressure to the max @ about 20 degrees after TDC as short after piston speed will hit the max able to convert all this combustion pressure over to the crank and conn rod as crank and conn rod are @ a 90 degree angle short after 20 degrees ATDC.

Also end of injection strongly depend on the load condition and engine RPM of the engine. In fact it can be on way before TDC or way after even to kick the turbocharger to live or a DPF that need some temperature. In general we could say introducing a large diesel fuel mass in a very short time into the combustion chamber will give a bigger delay before max combustion temp/pressure is hit but will give far more peak pressure that’s sustained a lot longer as well so end result is more power output. Also as there is more delay on hitting max burn rate of fuel as we need to evaporate the large fuel mass so you are able to advance start of injection a bit more so more time is created to extend injection duration and what we have seen so far is diesel injection ends @ TDC or short after on hi load conditions.

Example of engine development @ one of our customers using our development ECU systems where cylinder pressure and injection durations a placed in one screen to get a clears picture on what makes things chance.


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