why do diesels love turbos?

It seems almost all diesel engines have turbos. why?

Air is free?

It's just a way to get more power out of a smaller engine package. A displacement replacement, if you will.

I'd guess that since a diesel makes less HP per displacement than a gas motor, the diesel needs a little help. The 2.0 is about 110 hp, and the turbo 1.9 is only 90. Imagine what it would be w/o the turbo.

Gothmolly said:

I'd guess that since a diesel makes less HP per displacement than a gas motor, the diesel needs a little help. The 2.0 is about 110 hp, and the turbo 1.9 is only 90. Imagine what it would be w/o the turbo.

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The 1.9 SDI makes about 75 HP I think.

Try like 68, at least for the one in the Chinese-spec A2.7 Jetta GDF.

Anyway, diesels have lower EGTs, and are easier on turbos, as well, so they're easier to make for a diesel.

For the same power as a gaseer the diesel needs a lot more air to ensure the combustion is completed, so the turbo also contributes to lower CO emissions and smoke.

so the turbo makes the cumbustion leaner?

cp said:

Air is free?

It's just a way to get more power out of a smaller engine package. A displacement replacement, if you will.

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so why doesn't every honda 4 cylinder come with one? i'm certain the main reason is something else. air is free, but turbos aren't. there is something more essential about running them on a diesel than getting more power.

There's another reason that I thought of.

Gassers are rich burn. Run them too lean (read: too much air), and you'll melt a piston.

Diesels are lean burn. Run them lean, and you'll cool everything down. Score one for turbos - they actually help economy on a diesel, because you don't have to turn up the fuel accordingly.

Now, another factor. A turbo ups the compression ratio. Upping the compression ratio in a diesel helps the combustion, even on D2. Upping it in a gasser, however... you've gotta get higher octane fuel to prevent detonation.

Honda's philosophy is anti-turbo. PLus, it means the added expense of premium fuel.

veryhumid said:

so the turbo makes the cumbustion leaner?

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It's not a homogeneous mixture like you (should) find in a gasser. When the diesel fuel is sprayed in the droplets need oxgen to burn. It only used what it needs however to complete the combustion. The reason for the copeous amount of extra air is to improve the chances that all the fuel will be burnt.

Everybody is at least partially right
The TDI is lightly turbocharged compared to medium/heavy duty engines. Max boost pressure is 1/2 or less compared to M/HD engines. The TDI boost level keeps the air temperature below the coking temperature on the compressor wheel (which would reduce compressor efficiency and raise intake temperatures). Thus there is not too much concern about EGR gasses carrying oil to the comp wheel on a TDI...
Turbocharging, by providing additional air, effectively generated a leaner mixture for a given (steady state) power level. It also allows maximum power levels to be increased significantly by providing additional air to the combustion process. Best example I can offer is the naturally aspirated Cummins NHC 250 of the mid 1960s. This was the highest volume engine producing 250 HP @ 2100 RPM before the advent of turbocharging. This was a 14 liter engine. Now, my 2003 5.9 liter Dodge Ram 'B' Cummins engine produces 305 HP at 2900 RPM, and later engines are 325 HP. The nearest Cummins engine in size is the ISX 15 liter engeine and it is rated up to 600 HP last time I checked.
All of this is with reduced emissions. The NHC 250 NOX level was around 16 grams/BHP-hr; current engines are 2.5 grams or less. Obviously, there are improvements in combustion technology, injection processes, etc. that support this improvement, but turbocharging is a key element in the progression of Diesel technology.
John C

... but Honda's very good common-rail diesel engine in the European Accord is turbocharged.

Let's examine for a moment. A normally-aspirated diesel engine works. It has the same RPM limitations as any other diesel engine. It has to run lean (less fuel per revolution of the engine) and that means less torque. The higher thermal efficiency makes up somewhat, but gas engine efficiency at full load is not all that bad, so it's not enough to make up for squirting in less fuel. So now, for an engine of given size and weight, you have less torque and doubly-less power (less torque AND less RPM) compared to a gasoline engine.

The most advanced P-D version of the non-turbo 1.9 SDI (TDI without the T) makes 68 horsepower. Any halfway decent gasoline engine of that size will make TWICE as much. For example, the 2.0 FSI non-turbo makes 150 horsepower. (The extra displacement is obviously only a small piece of that.)

What to do? Can't increase the revs, diesel just won't do it. Solution - stuff more air into the cylinders to let it burn more fuel to make more power.

Supercharger? Bad move. The shaft power required to turn the supercharger eats up a good chunk of the power gain, and it goes against the whole objective of a diesel engine to be as efficient as possible.

So, you use the otherwise-wasted energy in the exhaust to stuff air into the engine. The most conventional way to do that ... is called a turbocharger.

It is not the ONLY way to do it. There is another fascinating device called the Comprex pressure-wave supercharger, which also accomplishes this. It is driven by the crankshaft, but only for purposes of timing itself, it does not use shaft power to compress the air. The Comprex is not subject to turbo lag and is not subject to being unable to make boost at low engine revs. It has other problems, though, which is why they have not seen large-scale production.

veryhumid said:

so why doesn't every honda 4 cylinder come with one? i'm certain the main reason is something else. air is free, but turbos aren't. there is something more essential about running them on a diesel than getting more power.

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That would decrease the rated MPG of the motor and wouldn't really be needed for most folks. You can get 0-60 in less than 10 seconds with most (any?) I-4 NA 2.0 gasser (say around 120 HP), and still get high 30 MPG with that same motor. Throw a turbo on there and I bet the MPG drops to mid 20's (WRX for example) but you get 50% or more power.

Ah, the Comprex. Didn't Mazda use that in the 626 diesel in the early '90s or so?

Bob_Fout said:

That would decrease the rated MPG of the motor and wouldn't really be needed for most folks. You can get 0-60 in less than 10 seconds with most (any?) I-4 NA 2.0 gasser (say around 120 HP), and still get high 30 MPG with that same motor. Throw a turbo on there and I bet the MPG drops to mid 20's (WRX for example) but you get 50% or more power.

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I mostly said that to point out i didn't buy that the turbo was just added to give power to small engines.

bhtooefr said:

Now, another factor. A turbo ups the compression ratio. Upping the compression ratio in a diesel helps the combustion, even on D2. Upping it in a gasser, however... you've gotta get higher octane fuel to prevent detonation.

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the compression ratio is just a ratio of the distance between the extreme points traveled by the piston. turbo doesn't change that. It does compress the air fuel mixture, which gives you a more dense air/fuel mixture in the cumbustion chamber, but it doesn't change the compression of the engine. I get the part you said about diesels liking to run lean, that agrees with the fact that when you turbo a gas engine, it needs more fuel and MPG goes down.

It changes the actual compression pressure or effective compression ratio. Turbo diesels generally have actual compression ratios in the 16:1 to 18:1 range whereas non-turbo'd ones are typically 22:1 or more. Actual compression pressures end up about the same.

Diesels have had turbos long before anyone ever got interesed in reducing emissions. The Mack garbage trucks you see emitting great clouds of black smoke on initial acceleration are all turbocharged. Just having a turbo on an engine does NOT mean it is low emissions. It is simply a way of putting more air in the cylinder and if you put in more air you can put in more fuel. More air in a diesel means you can put in more fuel; it is optional. More air(flow) into a gas engine means you did put in more fuel--no option; you have to maintain the correct f/a ratio on a gas engine whereas f/a ratio on a diesel really has no meaning. More fuel = more power.

cp said:

It changes the actual compression pressure or effective compression ratio. Turbo diesels generally have actual compression ratios in the 16:1 to 18:1 range whereas non-turbo'd ones are typically 22:1 or more. Actual compression pressures end up about the same.

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sorry i'm a little confused. a turbo changes the effective compression ratio, NOT the actual ratio, correct?

wouldn't the turbo have the effect of increasing the effective compression?

Yes, it does. I thought that was what I said. Therefore turbo'd engines start out with lower mechanical compression ratios. If you just added a turbo to a 24:1 diesel, you wouldn't be able to run much boost because the pressure in the cylinder would be too high.

Keep in mind that turbos also work on many different levels.

1) They use the energy from the exhaust to put energy into the intake & make more power (gotta recall the actual physics properties).

2) They increase volumetric efficiency. VE mathematically describes how efficiently an engine is using the air that is moving through it. NA engines (RUG & D2/Bxx) operate about 50-70%; Turbos run about 80-90% or more.

The discussion in this thread has been talking around the fact that the more air you cram into a combustion chamber and the tighter you squeeze it, the more power you make from it. This is what makes the turbo have such an impact on a motor.

Diesels in particular, need to move more air and move it more efficiently. This is why turbo's work so well on diesel engines.

I'm just scratching the surface....need to dig out my turbo book!

HTH

Tony

now the technical question, exactly how much does the effective compression ratio increase with 15psi of "extra" compression


BTW, thank you everyone for responding to this thread, our forum has a lot of very informed members

Turbocharging effectively increases the compression ratio of the engine. Do very much turbocharging to a gasoline engine and you are going to need higher octane, more expensive fuel to prevent pre-ignition. Additionally, the engine would have to be built stronger and heavier to handle the increased cylinder pressures. More weight to carry around.

Diesel engines with the direct injection do not have this pre-ignition issue, as long as the injection timing is correct.

TW

TurbineWhine said:

Turbocharging effectively increases the compression ratio of the engine. Do very much turbocharging to a gasoline engine and you are going to need higher octane, more expensive fuel to prevent pre-ignition. Additionally, the engine would have to be built stronger and heavier to handle the increased cylinder pressures. More weight to carry around.

Diesel engines with the direct injection do not have this pre-ignition issue, as long as the injection timing is correct.

TW

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Another way of saying this is that it changes the effective engine displacement?

yes, i think that makes sense. the ratio is intial volume/final volume correct?

DPM said:

Ah, the Comprex. Didn't Mazda use that in the 626 diesel in the early '90s or so?

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Yep, sure did. http://forum.mazda6tech.com/about3527.html