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How is the torque curve of a gasoline engine different from a diesel?

  1. Apr 23, 2013 #1
    Hi, my lecturer has given us two graphs showing the general shapes of the max torque curves for both gasoline and diesel engines, and they clearly show that the diesel engine has a much flatter torque curve than the gasoline engine, but not explained why.

    Expected to find this information easily with a Google search but struggling to find anything, can anyone explain the reasons for the differences between these two curves?

    https://dl.dropboxusercontent.com/u/630750/Screen%20Shot%202013-04-23%20at%2009.24.17.png [Broken]
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Apr 23, 2013 #2


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    I'm going to risk making a total fool of myself in public by attempting to address this issue without any creds to back it up. It's based solely upon my experience as a driver.
    A Diesel engine has an extremely high compression ratio (around 16:1, if I recall correctly). A currently typical gasoline engine is probably about 8:1. (My Roadrunner is 12.5:1, but that's a full-race motor and has to burn aviation fuel.) A gas engine uses spark plugs to ignite the air/fuel mixture, but in a Diesel it's the heat of compression that sets off the burn. Because of that ignition system, and a few other factors, a Diesel is most efficient at very low rpm's. That compression factor gives a tremendous down-force to the pistons when it goes "boom". The only time that I drove a semi, I was instructed to shift at 2,800 rpm. A gas engine typically shifts at around 5,000 - 7,000 rpm depending upon various factors (that's going by auto transmissions, which I hate). Some rice-rockets go to about 10,000 rpm, but those are just nuts. Anyhow, because of the compression factor, and often piston diameter, a gas engine has to turn a lot faster, and be geared down a lot, in order to produce the same sort of torque that a Diesel puts out naturally. Conversely, a Diesel has to have a lot shallower gearing in order to get any speed to the wheels.

    edit: I must admit that I didn't have my reading glasses on when I looked at your chart the first time. I see that you're pulling into the 4's with the oil-burner.
  4. Apr 23, 2013 #3
    As this is not numerically comparing torque output but the shape. As a primary factors, it's probably easiest to ignore the engine altogether to answer this question.

    The torque curve is exactly the same shape as the BMEP curve, which is indicative of how the engine is breathing. So the main shape of the torque curve will be defined by the induction.

    So what is different between a 'typical' diesel engine and a 'typical' petrol engine?

    As a secondary aspect, you should consider how each engine type controls load.

    EDIT: In this case, my first point isn't actually valid as I made an assumption that probably isn't true judging by the curve shape. I'll leave it in as it's a decent discussion point.

    It's also a bit of a bogus question as you can get virtually any shaped torque curve you want from either engine.
    Last edited: Apr 23, 2013
  5. Apr 23, 2013 #4

    jack action

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    When choosing the proper valve timing and intake & exhaust sizes for an engine, it will be tuned (or most efficient) at a single rpm (approximately where the maximum torque happens). As you will go further away from this «ideal» rpm (higher or lower), the torque curve will tend to drop. The further you are, the more noticeable the drop.

    If your rpm range is narrow (as with a diesel), the torque curve will seem flatter because you are always closer to your «ideal» rpm (which is logically situated close to the middle of the range in a typical engine).

    Still, be careful with the scale used in a graph; any peaky curve can be made flatter by simply changing the scale.
  6. Apr 27, 2013 #5
    No-one else think the gasoline and diesel titles are the wrong way round?

    Diesel typically peaks early and drops off hard. Gasoline can maintain a nice flat torque curve over a large portion of of the operating range.
  7. Apr 27, 2013 #6


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    Or it could be the other way around. I've seen more SI engines rev to 6500 rpm than CI engines.

    Diesels are typically designed with flat torque curves to make it easier for the vehicle to get going when hauling a heavy load. Many diesels are supercharged in addition to being fuel injected, and supercharging helps to flatten the torque curve at the low and high ends of the RPM range.

    Gasoline engines can see a peaky torque curve, because at high revs, pumping losses can cause a drop in getting air into the cylinders. Supercharging and fuel injection can help eliminate this in gas engines, like it does for diesels.
  8. Jan 7, 2014 #7
    A Diesel Piston Engine reaches its maximum torque at a much lower RPM than a Gasoline Piston Engine because a Diesel Piston Engine has a very high Brake Mean Effective Internal Cylinder Pressure (BMEP).

    A Four-Stroke Diesel Piston Engine only draws in air and then compresses this air to extremely high compression pressures during its compression stroke. Then with the aid of a glow plug (only during startup cranking) at Before Top Dead Center on the compression stroke a fuel injector injects diesel fuel directly into the highly compressed ignition temperature air lighting off the diesel piston engine.

    After a Four-Stroke Diesel Piston Engine lights off the glow plug is no longer needed because the ignition is supported by heat-of-air-compression. When air is compressed to such extremely high pressures the temperature of this highly compressed air also increases in temperature. The temperature of such extremely high pressure air is hot enough to ignite the diesel fuel all by itself without a spark plug or glow plug. This is also known as auto-ignition.

    Diesel fuel or Kerosene (Turbine Jet) fuel can only be used in a Diesel Piston Engine because these types of fuels are far less volatile compared to Gasoline. Under such extremely high compression a very stable and non-volatile oily fuel can only be used.

    The same types of fuels used in Turbine Jet Engines can be used in Diesel due to the high compression ratios both types of engines have. Diesel fuel is slightly heavier than Kerosene Jet fuel because it has more lubricants within its fuel type. But both Diesel and Kerosene fuel can be used in heat-of-air-compression or “auto-ignition” type turbine and diesel piston engines.

    Four-Stroke Diesel Piston Engines such as on a Land Rover operate at a compression ratio of 21:1! The compression of air to such extreme pressures will cause the air to reach a temperature sufficient to light off the fuel.

    A glow plug is only used during start up, once the diesel piston engine lights off the glow plug is no longer needed. The air compression alone is so extremely high that it develops enough heat from the compression alone to ignite the diesel fuel when the piston reaches before top dead center on the compression stroke. This is when fuel injection takes place into the ignition-temperature compressed air lighting off the diesel fuel and initiating the powerstroke.

    Diesel Engines are much like Turbine Jet Engine cores except instead of a turbine to absorb the hot gas expansion to power the engine a piston does instead. All that is needed to keep a Turbine Jet Engine running after it lights off is a continual supply of fuel. Also to shut off a Turbine Jet Engine all that is required is to shut down the fuel flow the same is so with a Diesel Piston Engine.

    Many commercial Four-Stroke Diesel Piston Engines have similar compression ratios as many commercial Turbine Jet Engines!
    Commercial turbine engine compressors operate at compression ratios from 20:1 to as high as 34:1.

    Since the BMEP pressure upon the piston head area is so extremely high in a Four-Stroke Diesel Piston Engine the maximum Brake Horsepower and Torque will be reached at much lower RPM.

    This explains why the graph for the torque curve is so much higher and stable while at a much lower RPM without much fluctuation.

    BMEP, Brake Horsepower and Torque are all directly proportional to each other.

    If for every 1 cubic inch displacement 1 Ft-Lbs of Torque is developed on a four-stroke piston engine then the BMEP is 150.8 PSI.

    If for every 1 cubic inch displacement 1 Ft-Lbs of Torque is developed on a two-stroke piston engine then the BMEP is 75.4 PSI.

    BMEP in PSI inside of a four-stroke piston engine:

    BMEP in PSI (4-Stroke Piston Engine) = [(150.8) x (Ft-Lbs Torque)] / [CID]

    CID = Total Cubic Inch Displacement

    BMEP in PSI inside of a two-stroke piston engine:

    BMEP in PSI (2-Stroke Piston Engine) = [(75.4) x (Ft-Lbs Torque)] / [CID]

    CID – Total Cubic Inch Displacement

    Volume Conversions:

    16.387 CC = 1 Cubic Inch

    1 CC = 0.061 Cubic Inch

    1,000 CC = 1 Liter

    Brake Horsepower (BHP) of a piston engine:

    BHP = [(BMEP) x (L) x (A) x (N) x (K)] / [(33,000)]

    BMEP = Brake Mean Effective Internal Cylinder Pressure per powerstroke in PSI
    L = Piston Stroke Length in Feet.
    A = Piston Head Area in Square Inches = [(Piston Diameter / 2)² x (Π)]
    N = Number of powerstrokes per minute ==> 4-Stroke = [(RPM) / (2)]
    2-Stroke = RPM
    K = Number of Cylinders

    Brake Horsepower = Usable horsepower at the crankshaft at a certain RPM after horsepower lost to keep engine running.

    BMEP = Average gas pressure exerted on piston head area during every powerstroke.

    Torque = Twisting moment at the crankshaft measured in Ft-Lbs.

    Power = Ft-Lbs per 1 second or 1 Ft-Lbs/Sec = 0.737 Watts.

    Horsepower = 550 Ft-Lbs/Sec or 746 Watts or 746 Joules/Sec.

    The easy way to understand piston engine usable power output is to know that BMEP, Torque and Brake Horsepower are all directly proportional to each other.

    Since the BMEP is extremely high in a Four-Stroke Diesel Piston Engine compared to a Four-Stroke Gasoline Piston Engine the maximum BHP and Torque will be reached at a much lower RPM in a Four-Stroke Diesel Piston Engine.

    More RPM past the torque curve equilibrium zone does not mean more Torque and BHP and therefore does not mean more BMEP either.

    The torque curve will remain high, stable and not deviate as much over a wider RPM range in a Four-Stroke Diesel Piston Engine compared to Four-Stroke Gasoline Piston Engine. That is because of the heat-of-air-compression auto-ignition nature of the Four-Stroke Diesel Piston Engine maintains very high BMEP and operates at a very high compression ratio.


    - MisterDynamics -

    January 08, 2014
  9. Feb 1, 2014 #8
    The diesel engine torque curve looks very similar to a supercharged gasoline engine torque curve. That made me think of another advantage the diesel engine has besides higher compression ratio, atmospheric pressure at the valve when it opens. The diesel engine is already more efficient without the throttle restriction. Something else not mentioned is the fuel, diesel has a higher energy density.
  10. Feb 1, 2014 #9


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    I think this is the true answer. The torque curve can be flat or peaky for either type of engine, depending on the design of the engine. I suspect that the racing diesel engine used in the Audi R10 Lemans Prototype race car is peaky compared to street oriented diesel engines. Links including engine specs:


  11. Feb 2, 2014 #10
    ok, very important thing is to know whatever it is turbocharged diesel or not.
    turbocharged engine allows you to regulate boost, and it can change the torque curve significantly. there are low boost diesel engines with flat torque curve from 1500 to 3000rpm, and there are hi boost engines where torque rapidly spikes at 1700-2000 and fall down quickly after around 4000rpm. similar to hi boost petrol engines, only shifted around 2000rpm lower
    I have seen dyno sheet (something like 2005 ford 2.2tdci 170hp) with maximum torque at 2500rpm, and maximum power at 3200rpm. official numbers was diesel standard 1750 and 4000rpm
    for turbocharged engine the turbocharger is the main determining factor in shape of torque curve.

    another important thing is, that SI engine is limited by fuel mixture it could ignite, knock resistance, and emissions it is allowed to produce (closely tied to mixture richness). that means, that power (and thus torque) is tightly tied to amount of air the engine can pump. so profiles of cams, size of valves, intake and exhaust manifolds etc. you can manipulate the torque by changing ignition timing, but you are limited by knock. knock is main problem in low revs, so this could be one factor in low low-end torque of petrol engine

    CI is (was) not that much limited, and (in good old days) it could run air/fuel ratio anything from 10:1 to 50:1 . the determining factor here is the amount of fuel injected. so you could easily "mask" some fluctuations in amount of air sucked to the engine and by that flatten the torque curve. there is no knock problem here to limit the low end torque

    also as somebody else said, you are working with much smaller rev range, so diesel engine is much closer to the "ideal point" (set by cams, manifolds etc)

    I honestly don't know know about the contribution of compression ration, but I don't think it is big.
    what I know is, that N/A diesel engines usually had much lower torque compared to the petrol engine of same capacity. last one I remember was VW 1.9SDi (CR 20:1) which has around 120-140Nm. at the same time, 2.0 petrol engine (CR 9:1) has maximum torque around 180-190Nm

    next thing for diesel engines was fuel injection. for old VW PD engines injection was determined by cams and torque curve was limited by this. modern CR engines are much more flexible. there may be much more influence here (pre and post injection, maximum fuel pressure - atomization for high revs... ), I don't know

    and yes, if you are not that interested in peak values, you can do anything you want with torque curve. either CI and SI
    (I don't remember the numbers exactly, so...)
    Last edited: Feb 2, 2014
  12. Feb 3, 2014 #11
    Why does a higher BMEP mean lower RPM?
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