Diesel Engines and Compression in relation to a Petrol Engine (say.. 7MGTE)

[vas85]

Hey every1...

Don't know how or why but I started looking up the history of engines, I was first interested in when the first supercharger / turbocharger was invented... but then found myself reading about the diesel engine.... now given that engine works off massive compression... as there is no spark in a diesel motor by principle, how don't their headgaskets BLOW?

How can it be that with our motor.. or others it can't handle the pressure... when a Diesel runs a much higher compression by leaps and bounds.

If the above ideas are wrong... what generally causes headgaskets to blow, besides starving it from water / heat...

 

[tissimo]

There is no fuel in the air under compression so no detonation

 

[Supracentral]

There is no fuel in the air under compression so no detonation

Idiocy... If you don't know, please don't put shit like this out here. You couldn't be more wrong.

That "clacking" noise diesels make? That's detonation.

A diesel detonates every time it fires.

Diesels are build to a different standard. (They are a lot tougher).

 

[adampecush]

yep, no detonation. A controlled explosion doesn't stress parts the way detonation (sharp, powerful, uncontrolled explosions) does.

 

[adampecush]

Idiocy... If you don't know, please don't put shit like this out here. You couldn't be more wrong.

That "clacking" noise diesels make? That's detonation.

A diesel detonates every time it fires.

Diesels are build to a different standard. (They are a lot tougher).

diesels burn a charge designed to ignite under pressure (unlike gasoline....comparing detonation of the two is like comparing apples and oranges) The fuel is also injected near the end of the compression stroke, so the timing can be controlled, and stresses minimized.

 

[Supracentral]

Edit: I was wrong.

 

[TurboWarrior]

Compression itself ignites the fuel in a diesel. Compression on diesels are high but they are built tough. With gas engines its air first, then fuel is added to compensate for the increase in air delivery. On a diesel fuel is added first. Blowing your hg from running lean cannot happen on a diesel.

But the hgs do blow. GM's 6.5L diesel is a disaster of a motor. They blow headgaskets and injector pumps easy.

Here is a good comparison read.
http://www.diesel-central.com/News/cackle.htm

 

[Asterix]

There are two very different concepts in internal combustion engines: spark ignition (SI) or compression ignition (CI). The compression ratio in a CI engine is somewhere around 20:1. Obviously, the structure needs to be able to handle this. Secondly, there is no throttle plate on a CI engine. That is, the engine is always sucking as much air as it can. Power is adjusted by metering fuel, so you can think of it as always running lean. In a SI engine, you spend a lot of effort to keep air/fuel as 15/1 for an ideal burn at all air flows (throttle positions). You'll probably never reach that in a CI engine. The best kind of CI engine is direct injection. Getting good atomization of the fuel is tough in order to get a clean burn without particulates.

Turbos make more sense on a CI engine because they're always spinning fast, since there's no throttle plate, so there's almost no lag.

I've not figured out the effective compression ratio given a known PSI boost from a turbo, but I'm sure it never goes much over 13:1, and certainly not over 15:1 for gasoline. Nitromethane is a different story...

Asterix

 

[tissimo]

Idiocy... If you don't know, please don't put shit like this out here. You couldn't be more wrong.

That "clacking" noise diesels make? That's detonation.

A diesel detonates every time it fires.

Diesels are build to a different standard. (They are a lot tougher).

Detonation is an uncontrolled expostion of the air fuel mixture, this does not happen in a diesel as there is no fuel to burn as the air is compressed. The Fuel is injected directly into the cylinder at or near tdc and burned from the heat of the compression. There is no uncontrolled expoition. So please if you dont know dont say anything. And get off your high horse.

 

[Supracentral]

I've not figured out the effective compression ratio given a known PSI boost from a turbo, but I'm sure it never goes much over 13:1, and certainly not over 15:1 for gasoline. Nitromethane is a different story...

I've done the math. It gets surprisingly higher than you think.

E = C((B / 14.7) + 1)

Where E= Effective Compression, B= boost psi, and C= Static compression. Also remember that 14.7 is equal to 1 bar of boost.

Let's do an example. Let's say we have a 2JZ-GTE with a 8.5:1 static compression ratio with T88 boosting at 28 psi.

Let's do the math.

E = 8.5((28 / 14.7) + 1)

17:1 effective compression ratio.

I've run up to 40 psi on high end race gasoline (VP Import to be specific) in my drag cars with effective CR's of 24:1...

Detonation is an uncontrolled expostion of the air fuel mixture, this does not happen in a diesel as there is no fuel to burn as the air is compressed. The Fuel is injected directly into the cylinder at or near tdc and burned from the heat of the compression. There is no uncontrolled expoition. So please if you dont know dont say anything. And get off your high horse.

We're splitting semantic hairs, but in effect you are correct when it comes down to the way diesel ignites. Diesel doesn't burn very well to start with and it won't explode in the same manner that gasoline does. I retract my original post.

I can admit when I'm incorrect, high horse notwithstanding....

 

[supraguru05]

without boreing you all with a long diatrab on how disel engines work let me clear up some things said in the thread


some modern disels have throttles some do not it depends on the configuration such as egr etc

some a compression ratio standpoint the reason gasoline engines cannot go as high in CR as disels in that the air and fuel premixes and is then compressed it is this compression that cannot allow the gasoline to preignite therefore you cant compress it that much or it heats up and preignites

since disels compress just air (with egr) they can compress it as much as they want to get the peak cylinder pressure they want

disels control the load on the engine with fueling

spark engines control it mainly with air and then for the amount of air the SI engine wants it gets a corrisponding fueling

FYI disels run a/f ratios all over the place and always to the right of stoich ie lean

i work in the combustion performance group at a disel company so if you think i said something wrong odds are your misunderstanding it

 

[Dirgle]

My question is, why are the diesels in our trucks (Ford, Chevy, Dodge) so damn noisy when diesels in the Europeans cars are so much quieter. I've been told the noise you hear is the mechanical injectors, but that doesn't make sense on todays electronic fuel injection motors. My only guess is that the "knock" is much louder on the truck motors because of the longer stroke. The sudden combustion as fuel is injected creates a loud knock because of the effort that the flame front must over come to get the longer stroke moving, where as the cars have a shorter stroke, so the flame front just pushes the piston down, rather than slamming head long into it as with their bigger brothers. This is just a guess on my part. I have a pretty good grasp on the theory of diesel engines, but if somebody could answer my question who has had practical experience, that would be awesome.

 

[Doward]

Why don't you normally see diesels over 3000 rpm? Is it due to timing issues, fueling issues, or what?

 

[starscream5000]

Why don't you normally see diesels over 3000 rpm? Is it due to timing issues, fueling issues, or what?

This is just a guess here, but I'd say it has more to do with the limits of fuel injector than it does with the strength of the engine...

 

[Supracentral]

There's no REASON to spin a diesel any faster. Why beat the crap out of the motor if you don't need to.

If we could get those torque yields out of gasoline at 3,000 RPM's we'd be setting redlines like that too.

 

[adampecush]

This is just a guess here, but I'd say it has more to do with the limits of fuel injector than it does with the strength of the engine...

It was more of an electronic control issue rather than the injector itself. Modern control systems can fire the injector upwards of 17 seconds per combustion event - Apparently the new duramax diesels no longer have the typical "knock" due to improved fuel control.

Either way, if you can make 850 lb-ft of torque at 1500 rpm, why introduce the stresses and design constraints of spinning the motor any higher than needed? (edit...which is exactly what supracentral just stated)

 

[werewolves182]

In addition to what has already been said about spinning any faster, wouldn't the mean piston speed being too high at any higher rpms be a factor. Simply because the stroke is so much longer then any gasoline engine.

Please correct me if im wrong as this is just a guess.

 

[tissimo]

My question is, why are the diesels in our trucks (Ford, Chevy, Dodge) so damn noisy when diesels in the Europeans cars are so much quieter. I've been told the noise you hear is the mechanical injectors, but that doesn't make sense on todays electronic fuel injection motors. My only guess is that the "knock" is much louder on the truck motors because of the longer stroke. The sudden combustion as fuel is injected creates a loud knock because of the effort that the flame front must over come to get the longer stroke moving, where as the cars have a shorter stroke, so the flame front just pushes the piston down, rather than slamming head long into it as with their bigger brothers. This is just a guess on my part. I have a pretty good grasp on the theory of diesel engines, but if somebody could answer my question who has had practical experience, that would be awesome.

has to due with lag timing on the fuel burn.. more lag more fuel ignites sending a larger pressure wave towards the piston making the 'knock sound'

Reason Most diesels dont spin high is due to the Massive parts involved. Take a look at the common diesel engines, and their beefy compenents.. thats a lot of weight to swing around.. you gotta handle 20:1 compression + boost and then the power generated with it

But also does have to do with the timing, when injecting the fuel directly into the cyl theres a very short window to work with on higher revs hard to get all the fuel you need..

 

[Doward]

Yep. I was thinking a small displacement, higher revving diesel would be the ticket for an econo-box.

 

[TurboWarrior]

4 cyl diesel vw's still redline at 4000 or just above. They still have plenty of torque. So much torque in fact that you don't even need to give it any gas in a standard when letting off the clutch.

My question is, why are the diesels in our trucks (Ford, Chevy, Dodge) so damn noisy when diesels in the Europeans cars are so much quieter.

The least noisy diesel ive been around are fords. They have pilot injection I think. Im not sure how it works. But they are much quieter than most. Im sure different companys have different systems to make them quieter. The newer izuzu diesels in GMs are quieter than the old 6.5l diesels. You should have earplugs when working under the hood of those things.

and i had a friend with a vw jetta diesel. He installed a lightened flywheel.....it was 2x louder after that. So some of the sound dampening must be in things like that.