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premixed combustion phase; why does it spike down after it ignites?

Diesel Tuning Fundamentals

Discussion and questions related to the course Diesel Tuning Fundamentals


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Excellent course, I could learn a lot about the diesel engine. But one point I don't understand so far is: What makes the heat release decline so rapidly after it has once been ignited in the premix combustion phase? It would seem logically to me that after it once had been ignited that it would combust all the fuel that gets into the engine in one continuous process.

I thought this is probably a tricky question :)

The ignition delay will change the premix's peak, the longer the ignition delay the higher the peak typically. You are looking at the Premix injection peak and asking why it drops so quickly. Well the reason it drops so quickly is because the main injection is being sprayed during that time so it's heat release drops significantly. All that fuel being sprayed in the cylinder and atomizing starts absorbing a lot of heat very quickly. Not to mention the cyinder walls start to absorb heat as well. You are also seeing the piston get closer to TDC where it's no longer rapidly increasing compressing generating a lot of heat.

Premixed combustion phase very well explained by JaegerWrenching however it differs a lot depending on engine model and fuel injection system and in general it will not make it an easy job to dial in all the correct numbers to get diesel combustion working for you so most will stick to old school type of injection as a basic benchmark.

So what to look for? We think in rate of burn and burn delay. As you know a diesel engine dos not have a active ignition source like a sparkplugs on a petrol engine so getting better control over the combustion process will give better engine performance as for more power and lower fuel consumption and less smoke. Getting better control over the combustion process has nothing to do with your engine management system. Type of fuel system and engine configuration will make the difference and therefore Premixed combustion phase will be very different as well so things like you end of injection on full engine load must be on TDC or short after is a load of nonsense on modern diesel engines like Euro5/6 Stage 5 or tier 4 final. So how come?

4 things in general. 1 compression ratio dynamic & static as for end of compression stroke temperature. 2 injector nozzle and fuel pressure. 3 combustion chamber. 4 swirl or no swirl.

Old school combustion. Let cook diesel and low heat signature in combustion chamber.

As for hi CR. Hard to control Premixed combustion phase as start of injection becomes critical under boost. Also hard to cold start without pilot injection and diesel rattle on low engine load/low boost. Relative hi thermal spike but massive drop in temp after injected fuel quantity start evaporating extracting a load of heat out of the combustion chamber. Hi flow injectors not recommended.

As for lo CR and hi flow injectors. More control over Premixed combustion phase but may need more advanced start of injection. Relative lo thermal spike @ Premixed combustion phase giving slower rate of burn on start of combustion but more gentle combustion build-up in a later phase around TDC and end of injection should be around TDC as well.

Modern diesel combustion. Do not know if this will apply to small bore hi speed diesel engines running alloy pistons.

Same idea with hi and low CR. Hi CR is even a lot more intolerant on control over Premixed combustion phase. In fact it start looking like a petrol engine running hi boost on hi CR ratio only difference is it dos not detonate but build massive combustion pressure very fast lifting the cylinder head when this happens @ TDC.

Lets burn it fast with hi heat signature in the combustion chamber. In this combustion type it gets very hot so we like to use steel pistons only as alloy will not last long as for massive thermal stress on them cracking topside on the piston dome many due to temp difference on the top side piston and deeper piston dome that’s cooled by the piston cooler using oil.

Premixed combustion phase on hi engine load can be as low as 5 crank degrees so how do they do this. Injector nozzle and fuel pressure. Multiple holes in Nozzle (up to 10 holes possible) and hi quality spray pattern and hi fuel pressure >2500Bar will make very small diesel droplets so very fast atomizing possible. Critical are the combustion chamber as for not hitting the relative cold piston dome with diesel spray but blasting the combustion upwards as expansion/combustion takes place above the piston and most type of modern injectors spray @ a very shallow angle under the cylinder head so that where you like to maintain your combustion as well. Hi swirl or no swirl. Hi swirl can disturb your diesel tray patron as it’s a fine mist and you do not want to mix these fine droplet making them into bigger droplets.

Result you will get a very hi rate of burn and fast heat build-up from start of combustion so as the Premixed combustion phase is almost not existing we can extend injection duration to about 20 ATDC so we can now move from max 30 crank degrees of injection for older diesel injection system to 50 crank degrees of injection with Modern diesel combustion prosses following the combustion by adding more fuel during the combustion.

As for fuel model. Most focus here will be on 3D tuning only as for very basic diesel tuning. Real live on top end performance diesel engines need a load more and question will come up on how can I get max power/efficiency while you need to waste some combustion energy to spool the turbo as with every load request end up with minimum ideal end of compression temp done by boost pressure. And as for fuel what timing/rail pressure to run?? As for a 3D fuel model this will not be possible so that why we have different running modes for static and dynamic load request build in our hi end ECU systems but 3D tuning will do the job for most out here I think.

Get that fuel model working on a modern engine and you end up with 10% fuel reduction and 20% more power out of the same engine capacity but as a lot more energy end up on the crank exhaust gas volume/expansion is also a load less so it’s a give an take with a new challenge to spool the turbo adding more complexity to the fuel model. For those running a VGT turbo most will spool up easy any way so 3D tuning will do the job well there. As for free floating turbo needing good low end performance. Not easy.

As for AFR is just a number it dos not tell you a lot as you can have smoke even with a 18:1 AFR running out of injector duration as for to retarded end of injection so getting to know the number on injection angle and start of injection tell a better story but is there a simply story to this as some will like to tell you?? NO as it’s to dependent on engine model and fuel injection hardware used. As you can read above 10% less fuel 20% more power only possible by changes in technology done in the last 5 years just by better control on Premixed combustion phase so that’s a big step and no wonder that we are not investing time on older engines upgrading them as for the same effort we can get the same power numbers with less fuel needed.