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diesel vs gas

diesel vs gas

diesel vs gas

(OP)
has anyone done a comparison or know what the differences in cylinder press and cylinder temps on a diesel engine vs SI engine of comparable size?

RE: diesel vs gas

MANY books and articles discuss this - try Google

RE: diesel vs gas

Your criteria are too vague.  You need to narrow down the application, and how much of an apples-to-apples comparison you are looking for.

RE: diesel vs gas

Geez, I wish somebody had though of this before (you got your homework assignment).

RE: diesel vs gas

(OP)
I am a engineer.  I am not looking for a detailed report on the differences with temp and pressure graphs.  All I am asking for, is if someone has experience with both types of engines to say engines of comperable size and power rating. ie: "Diesel cylinder pressures are about 10 to 15% higher and temps are about 20-30% higher."

  I am familiar with gas engines but never took the time to investigate diesel engines.  This question was more for curiosity than anything else.  I don't know enough about diesel engines to know if there are any comporable sized engines( same size and power rating).

Theoritically for the same size engine, with same bore and stroke temps and pressure should be the same.  However, diesels use the compression to ignite the feul.  So does the diesels have slightly pressures for the same engine or are they significantly higher?  Also if the diesel is smaller to make the same power of a larger gas engine, is it the increased pressure from compression ignition that allows the bore to be smaller.   
 
Are the same pressures seen in the cylinder because the diesel feul reguires more compression pressure to ignite it and the gas engine just simply does the same by using a spark.  

Again this was meant for general discussion.  Not a specific project im working on and don't need the pressure and temp graphs.

RE: diesel vs gas

small SI engines usually can get to about 10:1 or 11:1 compression ratios on conventional gasoline before you start having problems with autoignition.

small CI engines running on No. 2 Fuel oil (standard diesel fuel) run anywhere from 15:1 up to 25:1. Diesel engines typically see higher cylinder pressures (not sure about temperatures) which is why diesel engines, in general, are more robust in their design.

 

RE: diesel vs gas

If they have higher pressures, they must also have higher temperatures as temperature and pressure are directly related as expressed in the combined gas laws.

A hint will be the rpm they make their power at. If two engines are the same dimensions and make the same power, but one does it at substantially lower rpm, the lower rpm engine will also be the high cylinder pressure engine.

 

Regards
Pat
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RE: diesel vs gas

Diesel was FIRST!!

RE: diesel vs gas

Didn't Chevy (briefly) make a 350 diesel? That would probably be a pretty direct comparison.

RE: diesel vs gas

I think the 350 Chevvie was a pretty good SI engine for the day and age, but a pretty crappy diesel, mainly because it was not robust enough to withstand the pressures generated in a diesel.

Regards
Pat
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RE: diesel vs gas

Actually, it was Oldsmobile.  The Olds 350 was a very stout small block (basically the same casting as the 455), but obviously not enough.  Another shortcoming was the use of a fuel injection pump that was fuel lubricated.  A typical failure was water in the fuel damaging the pump, followed by the pump sticking and over injecting one or more cylinders, blowing the head gasket.  The Olds 350 ran between 9-1 to 10.5-1 in it's heyday and the diesel if I remember was 18-1.  I cannot think of any other direct comparisons, but typically a diesel makes more torque at a lower rpm and therefore less peak horsepower, so pat's rule would dictate a higher compression.  In fact, I've don't remember seeing a SI engine with more than 12-1 (Chevy L-88)or a CI engine with less than 16-1 (my current project, a Pielstick PA6B).

RE: diesel vs gas

Some pretty typical numbers - a commercial diesel engine will run (at some loaded value) around twice the compression, and per unit volume maybe 4 times the moles of cylinder gas, and maybe 2.5 to 3 times the mass of fuel. The peak combustion pressures and temperatures are both much higher than a gasoline engine. Not as high as those numbers work out to in the raw math, because if you allowed that to happen you would break something, melt something, or produce prohibitive amounts of NOx. Therefore, among other things, you retard the fuel timing, introduce inert gases, and intercool the compressed air - largely to reduce the peak temperature. Around loaded values, diesels will exhibit a tendency for a flattened peak combustion temperature that is near the point where the NOx creation takes off.

RE: diesel vs gas

My experience developing SI lean burn turbocharged natural gas engines from base diesel engines is that the peak pressures and temperatures are not that different, for the same power rating.  The diesels have slightly higher compression ratio, and the gas engines run a little richer.  The material limits are the same, typically, although the diesels do get pushed to higher ratings with commensurately upgraded materials; this is no longer apples-to-apples.

RE: diesel vs gas

Greg, that is interesting,  VW didn't seem to change much.  What problems did you encounter?

RE: diesel vs gas

Ecessive cooling of the cylinder head, leading to excessive smoking and poor fuel consumption. This was 30 years ago, admittedly.

5 years later they did manage to do it on a different block by working closely with a diesel engine manufacturer.

Cheers

Greg Locock


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RE: diesel vs gas

Generic question, generic answer. For a certain automotive turbodiesel engine that I have some familiarity with, I understand that the engine is designed for a peak cylinder pressure of 185 bar. This is some three times higher than a typical peak cylinder pressure for a normally-aspirated gasoline engine. Obviously there is a lot of give or take depending on the specific engine design that one picks for comparison, on either side of the comparison. In other words, "your mileage may vary".

Peak temperature is probably in the same range. The diesel has higher compression ratio and has boost pressure without perfect intercooling, so the temperature at the end of compression stroke is certainly higher. But the diesel runs leaner (less fuel in the charge means more dilution) and it's designed so that the moment of ignition - not the same as the moment of fuel injection! - is near or slightly after top-dead-center, as opposed to being quite a bit before for the gasoline engine. This is for both noise control and for emissions (and also, likely to prevent cylinder pressure from going through the roof).

For emissions reasons, newer diesels often run much more dilution with significant exhaust-gas recirculation even near full load, to try to reduce the peak temperature in the interest of reducing NOx.

RE: diesel vs gas

If they are being motored and of course depending on the compression ratio, most normally the CI is higher.
Some SI's can have the same or higher CR as a CI.
At unloaded idle the CI runs cooler due to the nonhomogeneous fuel air mixture.
The type of fuel makes a difference as well.

RE: diesel vs gas


  CI mostly runs cooler due to its greater 18:1 (or thereabouts) expansion ratio.            

RE: diesel vs gas

CI runs cooler because more of the energy is going to work rather than heat, and because there is so much inert material to absorb the heat energy. At load, they can run very high exhaust temperatures.

Expansion ratio and nonhomogeneous fuel air mixtures are not the correct way to view this. Expansion ratio is incorrect because you also have equivalently high compression ratio, more than offsetting this effect.  The nonhomogeneous mixture is not correct because it is irrelevant to the final bulk temperature after mixing.

The type of fuel can certainly make a difference. Primary effects there are different fuel heat energies and air-fuel ratios.

RE: diesel vs gas


  More energy goes to work because of the greater expansion ratio.

RE: diesel vs gas

BigClive, I'm not sure what you mean by that, but without modification that is a misleading way to think about it. The single greatest effect is that you have a more efficient heat engine from a greater temperature difference enabled by the high compression ratio.  Since one side of this is expansion, perhaps that is what you mean. However, since there is generally not an intentional increased expansion ratio, a normal compression ignition engine is not considered an engine having a greater expansion ratio.

Your post is suggestive that an Atkinson-cycle type of approach is causing some efficiency gain in a normal compression ignition engine. However, a higher compression ratio engine will be more efficient than a lower compression ratio engine, all other things being equal, including the expansion ratio being roughly the same as the compression ratio as normally occurs.  Further gains might be obtained with an Atkinson or Miller cycle approach, but that is independent of base compression ratio, and can be applied to either low or high compression engines.

RE: diesel vs gas

The Company I used to work for made and ran a successful CI engine with a CR of around 10:1. But it used very high boost to compensate.

RE: diesel vs gas

@PEW - I bet starting that thing was a challenge.

RE: diesel vs gas

The efficiency improvement with Atkinson cycle varies substantially with CR. Heywood has a nice section on what he calls 'over expanded' engines. The tradeoff of course is that WOT torque is reduced.

Cheers

Greg Locock


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RE: diesel vs gas

Greg,
I agree. I was emphasizing that a high compression ratio is not a synonym for, and does not imply, a manipulated expansion ratio.

RE: diesel vs gas

JSTEVE2,

Starting wasn't straightforward. The engines needed preheat to working temperature and the vehicle was push started but once on boost it took the world speed record for a diesel powered car. The company is  JCB and the vehicle is called DieselMax.

RE: diesel vs gas

Also, even with low compression from piston displacement, if it has a positive displacement, like a roots blower for instance, that might somewhat increase total compression and therefore charge temperature.

Regards
Pat
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RE: diesel vs gas

JSteve2 – on the subject of expansion ratios etc.
If I can I like to be able to visualise in a commonsense way the thermodynamic principles behind the various types of engine cycle.   As I see it the variation in CR is not the controlling factor in Otto cycle efficiency – it is the ER.   As I see it, the temperature after compression alone (that is; without the burning of fuel) is not a "real" temp gain – after the following expansion stroke the temp returns to its initial value.  The final temp after compression and combustion is the sum of the temp from compression plus the temp increase from the burning fuel.  Increasing the CR may raise the final compression+combustion temp but after expansion the temp rise due to compression cancels itself out.  Thus, the way I see it, only the heat gain from combustion is subject to the ER – the increased CR is merely a way of exposing the heat released by combustion to an increased ER.  The heat released by combustion would seem to me to be independent of CR etc.
  As one of my old thermodynamics textbooks remarks after deriving the formula for Otto cycle efficiency – "It will be noticed that the only variable in this equation is the expansion ratio r".   It seems to make a point of not saying "compression ratio r".   
  Does the increasing the CR raise the peak cylinder temp by more than that which results from compression alone?   I don't see how it could.
  The above is more of a suggestion than a statement – I am not totally sure about what I have written – perhaps you have a different view on these matters.
 

RE: diesel vs gas

BigClive,
I don't think there's anything technically incorrect with what you are saying.

Here is why I wouldn't state things that way - first if I make an engine with high CR relative to low CR, and run the thing without fueling it, the exhaust temperature of the high CR is higher than the low CR. Entropy dictates that less useful work is performed, unless that heat is what you were after.

Second, the efficiency gains are due to the Carnot temperature differences. Remember that, although doubling compression only doubles temperature (even less, actually), it also increases the Carnot efficiency meaning you can get more useful work out of the combustion. There are also some ancillary temperature effects, like reaction rate positive feedback, that drive your peak temperatures higher than might be indicated by the CR alone.  So you affect the high side of Carnot with the high CR. You can affect the low side of Carnot with a high ER, but note that for a nominal engine, as stated above, the diesel will result in a higher post ER temperature (all other things being equal such as the same charge temperature and moles). Atkinson tries to go deeper, but that's not what I see as the thrust of this discussion.

OK, so the useful work is available due to Carnot, but you have to recapture it mechanically in some fashion, and that happens during the expansion period, so that could be a way to state this. I just think it's odd to think of this as an expansion ratio effect when the ER relative to CR is the same for a nominal gasoline or diesel. However, as I stated, I'm not sure anything you are saying is technically incorrect.

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