Diesel Lower Egt Than Gas?? How??

[BkzProblem]

Ok guys, I've been reading stuff on here for some time now... one of the things that I have noticed that has caught my attention is in the holset turbos dicussions. I have seen more than 3 ocassions where people say that diesel engine egts are less than gasoline engines... this makes no sense to me... diesels rum hotter egts due to the fact thet they run on high compression alone.... Diesels can idle at an a/f of 100:1 because they make most of their power off of heat due to compression.... vs gas that uses spark plugs to help ignite the fuel... On diesel engines, the more fuel that is flowed into an engine, the hotter it gets... on a gasoline engine, more fuel is injected to help keep combustion temps down therefore reducing the egts... Diesels rely on heat to run so I don't get how you can say, holset turbos spool so fast because they were designed for diesels with lower egts, when I have a friend with a 5.9 6 cylinder cummins and his turbo won't hit full boost until he fuels the shit out of it and his egts hit around 1000 to 1200 degrees farenheight under full boost... I have never seen a gasoline motor hit even near 2000 degrees under full boost.... Don't get me wrong, I still support holst turbos due to the fact that I personally know of two people that have holset hx40 turbos badman21, and my cousin with a turbo 2.0 sohc 420a neon making 430 hp and is hitting full boost by 3500 rpm.. the least and now I have joined the bandwagon and bought an 7 blade hx35 with a 12cm housing to put in my 4g63 swapped mirage... but please, somebody clear this up for me.....

 

[Burtismaximus]

thats wierd my egt gives me 1300 degrees normally. so i dunno what to tell ya bud.

 

[BoredDSM]

Very hard concept to grasp. From what I understand this is how it works. To make gasoline you need crude oil. The oil is heated to different temperatures. First they heat it once at a low temperature and the fumes etc is collected and that turns into diesel gas. Diesel has a lower boiling temperature than say 100 octane. The reason higher octane fuel is used is because it will not ignite from the heat produced by compression. Thus why when you run higher compressions you need a fuel that won't ignite in low temperatures. A spark is pure energy and a lot hotter than the heat is produced from compression. When you add more fuel in a combustion engine, there is always more air to accommodate it which makes it burn hotter. Yes, if you run rich which means your running a higher fuel to air ratio your temps will be lower because of the fuel cooling everything down. Very complicated process but you should really look into googling this stuff and see what you can learn.

 

[KyleJ]

Diesel = 30lbs. of boost. Also, if a Cummins 12 valve goes to 1600* EGT's for more than a couple seconds, that motor is DONE!

 

[cbrue122]

When they say diesels have a lower egt, it more than likely means the average egt over its entire operating range. Diesels do not have a throttle plate limiting the amount of air getting into its cylinders, therefore at idle and light load it has the extremely lean burns of 50:1, 100:1, etc. These lean burns move so far into the lean category that the excess air starts to act like the extra fuel on dsm engines, resulting in cooler egts. So, for comparable engine loads, a diesel will have lower a egt than a gasoline engine.

I personally love the design and operation of diesel engines and really dislike how our dsm engines have to use excess fuel instead of air to cool combustion temperatures. But there is some promise in the new direct injection gas technology starting to emerge.

 

[mattdev]

]
I personally love the design and operation of diesel engines and really dislike how our dsm engines have to use excess fuel instead of air to cool combustion temperatures. But there is some promise in the new direct injection gas technology starting to emerge.

Doesn't that stand for all gasoline engines? Correct me if I'm wrong, but the oxygen is what burns, gasoline is just a fuel that helps the o2 burn. Adding more air, makes it burn hotter, no matter what? (since it is a more powerful combustion)

 

[BkzProblem]

When they say diesels have a lower egt, it more than likely means the average egt over its entire operating range. Diesels do not have a throttle plate limiting the amount of air getting into its cylinders, therefore at idle and light load it has the extremely lean burns of 50:1, 100:1, etc. These lean burns move so far into the lean category that the excess air starts to act like the extra fuel on dsm engines, resulting in cooler egts. So, for comparable engine loads, a diesel will have lower a egt than a gasoline engine.

I personally love the design and operation of diesel engines and really dislike how our dsm engines have to use excess fuel instead of air to cool combustion temperatures. But there is some promise in the new direct injection gas technology starting to emerge.

I totally agree wit you.. But in terms of turbocharging, diesels make almost no power when they are in the 100:1 range... they only make power when the ratio drops and a lot more fuel is dumped into the mixture and it increases the a/f/r....

 

[crimsondragon]

Doesn't that stand for all gasoline engines? Correct me if I'm wrong, but the oxygen is what burns, gasoline is just a fuel that helps the o2 burn. Adding more air, makes it burn hotter, no matter what? (since it is a more powerful combustion)

Wrong. Oxygen is not flammable. Therefore it will not burn. Oxygen mixed with petroleum products will self ignite however. That is why when you service and oxygen tank, you are never to have any type of petroleum product within the vicinity. This is for pure oxygen by the way.

I'll get technical for better understanding. Burning is nothing more than a chemical reaction. Without the mix of gas, oxygen, or heat, neither will ignite on it's own. However, mixed together, and you have formed something that will create a reaction, which is burning. I forgot the exact chemical composition for gasoline but typically when they react, they break apart, give off thermal energy, and create new elements, which is CO, CO2, and various forms of nitrogen oxide.

 

[cantrun10s]

I thought I'd throw in my 2 cents, just because my daily driver at the moment is a built 7.3 Powerstroke, and I like to think I know quite a bit about the theory behind the diesel engine. Skipping most of the subtle reasons Diesel EGT's would be lower, heres the one I think to be the biggest.

In a gasoline engine, your compressed mix of fuel and air is ignited at one point, and the burn travels outward from that point on. As soon as the piston starts to come back up after the power phase, the exhaust valves open and you push the (most of the time) still burning mix thru the exhaust to the turbo. Hot stuff.

In a diesel engine this isn't the case. Diesel is forced into a burn by the heat generated by high compression, meaning that instead of a flame travelling from a single point out, virtually all of the fuel and air goes off at the same time. By the time a diesel hits exhaust phase, there should be either no fuel(running good with no smoke), or no air(the ever popular overfueling just to make it smoke is still a mystery to me), remaining in the mix. It's just simply a more efficient way of igniting the mix and getting a full burn.

Anyway, that's my thought.

Oh and one more thing.

Diesel = 30lbs. of boost. Also, if a Cummins 12 valve goes to 1600* EGT's for more than a couple seconds, that motor is DONE!

30lbs is kinda.... low. On my Economy tune I pretty easily see 45 -50.

 

[cbrue122]

mattdev - you are correct in that running rich is not limited to just dsm engines; it applies to all gasoline engines. This fuel dumping method is just used to a higher degree in turbocharged engines like the 4g63. Engine designers have to do this for an added safety factor to keep the average driver from melting their turboed car; however, it makes us, the performance minded type, sad.

BkzProblem - true, diesels can't make power without the fuel.

Once you start putting big loads on a diesel is when egts need to be monitored, especially if you have it 'chipped' to dump more fuel. Diesel egt's can get just as hot if not hotter than a gasoline engine.

 

[JusMX141]

I have never seen a gasoline motor hit even near 1000 degrees under full boost....

Are you sure about that?

At 11.5:1 air fuel ratio on pump gas, I normally see around 1550*f.

 

[BkzProblem]

Ok.. Well someone answer this question... why is it that when you put diesel fuel in a car, it can't burn and it doesn't drive properly... and when you add gasoline into a diesel engine, it blows up because it starts to ignite before it even reaches the combustion chamber....
Diesel fuel needs a lot higher temp to be inginted vs gasoline... even with spark, diesel fuel can't be ignited in a gasoline engine.... the temps aren't hot enough.... I need valid reasoning 2 believe that diesel egts are lower than gasoline....

 

[PIMking]

My dumptrucks pyrometer shows me while loaded with 16 tons and wide open throttle at 30 psi of boost is 900* F..

I dont know why but I dont care. I watch that for my air filter. If its getting clogged my egt goes up higher.

 

[TunaTalon]

Diesel engines have lower EGTs than gasoline engines for the same reason that diesel engines are more energy efficient than gasoline engines. They have higher compression ratios that extract more of the energy from the combustion process.

Consider that when the piston of a 4G63 has gone down far enough that the volume in the cylinder has increased by a factor of 8.5 the power stroke is over and the energy remaining in the charge is exhausted. At that same point in a diesel engine with a 17:1 compression ratio the piston would continue down twice as far extracting more energy and lowering the EGT.

A simple rule to remember is that the less energy is wasted in exhaust heat the more efficient the engine.