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Gasoline engine - max torque and efficency

  1. Apr 23, 2014 #1
    I have the following question, intended for a car powered by a 4 cylinder, non turbo, gas engine.
    I would like to a rather conceptual answer, but anyway I will put some actual numbers.

    It´s said to achieve it´s max torque (200Nm) at 4000 RPM
    It´s max power is 150HP at 6500RPM
    I don´t know the fuel consumption curve, but i know that the best km/litre is about 115KM/h (3000RPM)
    At 4000RPM (max torque) the car runs at 130KM/h
    At 6500RPM (max power) you get 200Km/h

    Now, in a heavy load condition (five passengers, front wind and pulling a small trailer)... is it right to think that the best fuel efficency would be found at the max torque´s RPMs, lets say 4000RPM~130KM7h ?

    thank you very much.
     
  2. jcsd
  3. Apr 23, 2014 #2

    SteamKing

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    Possibly, but the opportunities for driving 130 kph are rather limited to interstate highways, autobahns, or motorways. It's probably not a good idea to be hauling a trailer around at this speed. Drive at a safe speed (or under the posted limit) and let fuel efficiency take care of itself. The lives of you and your passengers are not worth the few dollars you'll save driving like a bat out of hell.
     
  4. Apr 23, 2014 #3

    rcgldr

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    Max thermal efficiency (power output / potential power input based on fuel consumption rate) is usually highest at rpm of max torque. The issue is that most engines are producing more power at peak torque rpm than what a car needs for crusing speeds, and at higher speeds, since the drag increases with speed^2, then power loss increases with speed^3 (power loss = drag x speed). For a typical car, best fuel milage occurs around 60 kph to 80 kph.

    I'm not sure, but in a situation such as going uphill at some fixed speed, shifting to a lower gear and running at higher rpms may be more efficient (distance covered versus fuel consumed) than running in a higher gear at lower rpm and having to use a lot more throttle input.
     
  5. Apr 25, 2014 #4
    To answer your question, no.

    The combo will be most economical at the point where rolling resistance losses, parasitic losses, and internal friction match the power needed to overcome the wind resistance.

    There is a big difference between efficiency and economy.

    The thing to consider is that you are dealing with moving goal posts when it comes down to economy. Economy is going to be a balancing act between the amount of power (fuel) the engine needs to consume just to maintain a certain rpm (including rolling resistance and all the other internal frictions), v's wind resistance. Add wide open throttle (with the aid of a proper EGR) and you will have 'best' economy at a given road speed.

    It is best to visualise this with a BSFC (break specific fuel consumption) map for the engine, it will show you in grams per kw/hr of fuel required at a given load. The BSFC is nearly always best at 85-95% full load (not at 100 throttle as the ECU adds more fuel to protect the engine) at the lowest practical RPM where the cams are allowing the most inlet charge to stay in the chamber.

    Sorry about the sporadic post, my daughter deleted the first attempt.


    Damo
     
  6. Apr 25, 2014 #5

    SteamKing

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    BSFC = 'brake specific fuel consumption'
     
  7. Apr 26, 2014 #6
    Whoops! thanks Steamking.




    Damo
     
  8. Apr 30, 2014 #7
    Fuel economy is quite a difficult question.
    Yes, the engine will probably achieve highest efficiency ( = lowest BSFC) somewhere around revs of highest torque. that is when you are using your fuel most efficiently, but not when you use least of it !
    trouble is, that with that load (80-90%) and 4000rpm, your car will accelerate unless you will produce much more drag (for example with your trailer). but as aerodynamic looses increase with square of speed, you are burning huge amount of fuel just to go faster.
    if you would drive at 80-90kmh, the overall drag will be (more or less) half of that at 130kmh!

    so you will go slower. at 90kmh, you will rev around 2500rpm. engine will produce less torque and power, so you can cruise. yes, the engine will work less efficiently (higher BSFC) but as you don't need that much fuel to overcome high drag, your fuel consumption will be lower.

    BSFC = brake specific fuel consumption. it is the amount of fuel, required to produce power. for example you need 250g of fuel to produce 1kW for 1 hour. so at 4000rpm, 90% load (180Nm) engine is producing 75kW. so if you are driving one hour at that conditions, you car will burn 18.75kg of fuel. that is around 25liters. 25liters per 130km equals 19l / 100km.
    that is lot.
    in real life you are cruising at let say 40% load, so you have bit worse BSFC but also producing much less power.

    trouble is, that all this is theoretical.
    in reality:
    - your ECU may decide to do something completely different. my car (subaru legacy 3.0) is switching to open loop, if throttle exceeds 40%, engine speed exceeds 4000rpm or speed exceeds 130kmh. if that happens, engine is running richer, and fuel economy is getting worse.
    - at lower revs and high load your engine may knock. in that case your ECU will pull some timing (reducing torque/power) and/or add some fuel. = reducing efficiency
    - you don't know BSFC map of your engine. you engine may reach best efficiency at lower revs, or the efficiency may be only tiny bit worse at low revs. or not. we don't know.
    - outside conditions are changing (road inclination, wind, temperature..) which are forcing ECU to react and change timing/throttle opening/mixture to compensate.



    so most likely, you will reach best fuel consumption at lower speeds. from my experiences 70-80kmh.
     
  9. May 20, 2014 #8
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