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Internal combustion engines

  1. Mar 31, 2007 #1
    Hi everyone,

    We have an argument about gasoline combustion engine and diesel engine. Some say, the diesel engine is stronger than gasoline one. My idea is if the two engines are the same dimension, they must have the same power. Am I right or wrong. Thanks
     
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  3. Mar 31, 2007 #2

    brewnog

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    Wrong.

    The power developed depends on a lot more things than fuel type and cylinder capacity. There is no reason at all why two engines that have the same cylinder capacity should develop the same power; even engines of the same size and same fuel can develop vastly different powers depending on their intended application.

    Take a modified Honda S2000 engine; 2.0 litres developing 255bhp at 9,000rpm, and compare it with a Ford Duratec 2.0 litre engine developing 130bhp at 6,000 rpm. Both cracking Petrol engines, the same size, but worlds apart. Throw some agricultural Diesel engines into the mix developing 40bhp at 1500rpm and you'll see that there's a lot more to engines than capacity and fuel.
     
    Last edited: Mar 31, 2007
  4. Mar 31, 2007 #3
    Thanks for your answering me, but i am still in doubt.
    I just think the two are equivalent because i refer to some BMW models which is on the internet.
    For examples : the 630i with the gasoline 2996cm3 engine has the power of 190kw. The 320cd, diesel 2000cm3 engine has the power of 110kw.
    When comparing the power of the same engine's capacity, i assume everything else is the same, only different thing is petrol or diesel as fuel.
     
  5. Mar 31, 2007 #4

    brewnog

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    The power nodes are selected by marketing people and are not the maximum the engine could actually produce.

    You can't make comparisons between identical engines using different fuels, because the basic engine designs are totally different. If you put diesel in a petrol engine, it'll be far, far less powerful than a diesel engine using diesel. And vice versa.
     
  6. Mar 31, 2007 #5
    No, I don't put petrol into a diesel engine. I just want to ask if a 3 liter gas engine car and a 3 liter diesel engine car, the same make, let say BMW, which one is considered more powerful in general.
     
  7. Mar 31, 2007 #6

    russ_watters

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    The problem here, pixel01, is that there are too many variables in engine design to make this comparison. Even with one manufacturer - the different fuels have different requirements and the designs themselves have a lot of variables. About the only thing that can reasonably be said is that chemically, the energy content of diesel is higher.
     
  8. Mar 31, 2007 #7
    OK, I think i have to accept that. Anyway, after checking several models of BMW, i have found that, the ratio of power over cylinder's capacity is quite the same, a little bit higher for diesel engine compared to gas engine.
     
  9. Mar 31, 2007 #8

    brewnog

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    Pixel, as I said, this has very little to do with the engines themselves. The power figures quoted are those which the BMW marketing people think are most suited to the product. The power ratings you quoted are not limited by engine design or fuel; those engines (both petrol and Diesel) are easily capable of developing more power with no design changes at all; they are set to those kW figures for marketing reasons. They do not in any way reflect the relative power of petrol/Diesel.
     
  10. Mar 31, 2007 #9
    I think may be I did not express my question very well. I drive a number of motocycles : yamaha, suzuki, honda. I can see that the higher the cylinder capacity, the stronger the bike mostly regardless what make is that. A 600 bike certainly is stronger than a 450 one if they are both 4-stroke engine. A 2-stroke bike must be stronger than a 4-stroke if they are of the same capacity.
    Because they do not produce motorbike with diesel engines so I do not have the feeling between the two types. I do not drive a car, but i know that a 3L engine car must be stronger than a 2L one. That comes to a question, if, in general, two cars are of the same capacity, one is diesel, one is petrol, which one can produce more power at max.
    May be, if you drive a car, it is more difficult to feel the power of the car than you drive a motorbike.
     
  11. Mar 31, 2007 #10
    Diesel and gasoline motors do not even develop their peak power at the same point in the RPM curve. You can't just pick one and say it is better or develops more power.
     
  12. Mar 31, 2007 #11

    brewnog

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    Pixel, this is seriously misguided. A 2 stroke is not necessarily "stronger" (I think you mean "more powerful") than a 4 stroke of equal capacity. In general, yes, they have a higher power density. But this isn't a rule.

    Again, this is not necessarily true. Compare a 3.0 litre Jaguar with a 2.0 litre Subaru. Cubic capacity isn't everything. There is absolutely no reason why a 3L engine is inherently more powerful than a 2L engine. Some are, some aren't. Generally speaking, larger engines are more powerful than smaller ones because you can get burn more fuel in a bigger one. But there are dozens of other, more important factors affecting power output.

    Either, depending on the design. You can't speak "in general" terms here if you want a meaningful answer. If both are normally aspirated, you'd "generally" get more power from a gasoline engine partly because of the higher speeds they can run at. With a turbo Diesel, the difference is much less clear-cut.

    Absolutely not.

    Pixel, which is warmer, a cup of tea or a mug of coffee?
     
    Last edited: Mar 31, 2007
  13. Mar 31, 2007 #12
    May be there's some language misunderstanding, English is not my mother tongue. As I said, I drive motorcycles, in fact i am a motorbike fan, so there's no doubt that the bigger the engine, the stronger the bike, and the a 2-stroke bike normally stronger a 4 stroke. You mean it has higher power density. That is exactly what I mean. With a 2 stroke one, 124cc, you can accelerate much faster than a 124 4stroke. I consider that bike is stronger or more powerful.
    You compare a 3.0 litre Jaguar with a 2.0 litre Subaru and you said they are the same, but they are from different manufacturers and of course a lot more different things. If you compare 2 same make sedans 2.0L and 3.0L, they must be different. Of course I mean other conditions are also identical. If you go off-road, you need a 4 or 5.0L, not just a 1.0L car, because it must be strong enough to crawl up the steep hill.
    Why i said 'in general'. Because i do not know very much about cars and car engines, so in comparison I do not want to take into account the support of accessory equipment that make the engine faster or stronger. I mean all the conditions in comparison are the same except for fuel type.
    Some of mind friends who drive cars very often, sometimes tell me about this car is stronger that car, this is faster and one of the reasons they told me is the type of the fuel. That's why i started this thread.
    Anyway i thanks all of you for answering my question.
     
    Last edited: Mar 31, 2007
  14. Mar 31, 2007 #13

    brewnog

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    Pixel. You can't necessarily accelerate faster with a 124 2 stroke than with a 124 4 stroke. This is often the case, and there is a reason for it, but there's more to the issue than displacement. You can not make this generalising statement.

    No, I was saying they most certainly are not the same. The engines are designed to do different things, and the Subaru could easily outperform the Jaguar. While displacement is an issue, it is NOT EVERYTHING.

    Ok, by your admission, you don't know much about car engines. Feel free to take this opportunity to learn, or just keep restating your beliefs.

    YOU CAN NOT MAKE THIS COMPARISON. You can't just change the fuel type, engines designed to run on Diesel fuel are fundamentally different from those designed to run on gasoline. A typical Diesel car is fundamentally designed to suit a different market than a typical gasoline car. That's why trying to make this comparison is pointless and misguided.

    It's not all down to the fuel. I could make a Diesel engine outperform a similar petrol engine. Or vice versa.

    Look, in general, (speaking about passenger cars), a Diesel will have more low-end torque and better fuel economy than an equivalent gasoline car, which will have more top-end power. THIS IS NOT ALWAYS THE CASE, AND IS NOT A "RULE". Nor is it all to do with the fuel.


    Does the fact that there are two completely different types of fuel, and two different types of car available tell you something about the fact that there is no distinct winner?
     
    Last edited: Mar 31, 2007
  15. Mar 31, 2007 #14

    Stingray

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    While I agree with everything else you've said, why would this be true? Why wouldn't a manufacturer advertise as much power as their engine makes (and not more to avoid being sued)? Lots of cars are sold on their peak power figures.
     
  16. Mar 31, 2007 #15

    brewnog

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    Oh they do (or attempt to) advertise what the engine in that particular vehicle develops. But the decision as to what the engine develops is a marketing one; for example the 110bhp VW PD diesel engine is identical to the 90bhp and 130bhp versions, except for the engine management software. The limitation as to maximum power isn't on fueling/combustion/breathing, it's electronic, and it's a decision made to strike a balance between power, torque, fuel economy, emissions, insurance group, durability, reliability, and customer perception.

    Just because the engine is physically capable of developing lots more power doesn't mean it's in the manufacturers interests to sell it at that rating.
     
  17. Mar 31, 2007 #16

    Danger

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    This is also an inaccurate generalization. Off-road vehicles rely upon gearing, stance, tires, weight, etc. more than displacement. While my El Camino with 7.3 litres will climb over just about anything that a normally street-driven vehicle can hope to, I've seen <3.5 litre jobs that will climb over it while it's doing so.
    As the others have tried to point out, diesels and gas burners are absolutely not designed for the same purposes, and don't even work on the same mechanical principles. In general, a diesel of any given horsepower potential will provide far more low-end torque than a gas motor of equal or even greater hp. In a tug-of-war, a 150hp diesel truck will obliterate an identical vehicle with a 300hp gas engine. Conversely, the gas unit will totally trash the diesel in a 1/4-mile drag race.
     
  18. Apr 1, 2007 #17

    russ_watters

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    Just in case it wasn't clear from what others said, in general, if you take a diesel engine and put gas in it (or vice versa) it won't even run. They are that different in design. In particular, diesels don't have spark plugs and ignite the fuel by having a higher compression ratio than otto engines.
     
  19. Apr 1, 2007 #18
    I got this answer from an automotive engineer:
    "The Diesel will have more torque and get better fuel economy, the gas engine will have more horsepower and will be more responsive"

    Thanks all of you, and I certainly learn a lot from this topic.
     
    Last edited: Apr 1, 2007
  20. Apr 1, 2007 #19

    brewnog

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    Pixel, I work as a development engineer both on Diesel and gas engines. If you want to believe that statement and apply it to real life situations, that's up to you. But please don't use your sheer ignorance as an excuse for never having learnt anything from the people here who try and help you.
     
  21. Apr 2, 2007 #20
    You misunderstand me. I have said i learn a lot from this forum, and from this topic of course. I just put up here a simple answer from another point of view so everybody can see it and have idea. I did not claim you are wrong, not any, I just mean if you are that good, please try a simple way to explain and persuade me and of course many other members who have been visiting this topic. May be my weakness is the fact that I have never driven a diesel car and from theory to reallity there are distances.
     
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