Optimum engine RPM

Discussion in 'Start Your Journey Here' started by Niall, Dec 19, 2016.

  1. Niall

    Niall New Member

    I am a driving instructor with too much time on my hands and am wondering about what is the optimum engine speed for maximum efficiency.

    The general advice relates to the car speed (around 55mph), and to change gear at lowest engine speed tolerable.

    Coming from a motorbike background, I know BHP and Torque are important parameters for engine performance. To date I have not been able to locate any mention of the engine parameters' role in maximizing MPG

    Should the rpm where my car engine develops maximum power not be where I achieve maximum MPG?
  2. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    In my experience , the optimum RPM is zero. Wish I could help you more. Other folks here could give you a better answer , but depending on engine size and terrain , it could be 1400-1600 RPM.
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  3. S Keith

    S Keith Well-Known Member

    The short answer is "it depends".

    You have a massive number of variables.

    In almost every case, an engine is at optimal efficiency at its maximum power rating, i.e., If an engine puts out 150hp, it burns the least amount of fuel per horsepower when producing 150hp. Thus it's specific fuel consumption is the lowest. However, when does one need 150hp to drive in a fashion where best mpg is the goal? Never.

    When you start looking at low power cruise, i.e., 15-20 hp to keep a car rolling down the freeway on flat terrain, mpg is more a function of the amount of MASS flowing through the engine. Each volume of air inhaled on the intake stroke requires a charge of fuel to form the proper stoichiometric mixture for power, economy and emissions. More air means more fuel. Less air means less fuel. Of course, extremely low rpm throws a wrench into it as it may become very inefficient producing power at low rpms significantly increasing specific fuel consumption.

    The best answer is, whatever rpm your CVT settles on in steady-state conditions... :)
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  4. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Check out the Speed vs Fuel Economy charts here: http://www.cleanmpg.com/community/index.php?threads/49683/
    In nearly every case, slower is better.

    In my own car, the best steady cruise mileage (measured) is 5th gear at 25 mph, just barely above idle. It's probably actually lower than that, but that's as low as cruise control will work for a good measurement.
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  5. RedylC94

    RedylC94 Well-Known Member

    The first couple of sentences there are true, but the rest is invariably false. Look at any engine performance map that shows BSFC vs. speed and load.

    For gasoline automotive engines, peak thermal efficiency (minimum BSFC) typically occurs at less than half peak-power rpm and at moderately heavy load. However, peak engine efficiency in that sense doesn't necessarily coincide with best mpg of the complete car, because of variables outside the engine, like aero drag.
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  6. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

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  7. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Here's my recorded data on my car.
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  8. Niall

    Niall New Member

    Thanks for this, very illuminating.

    In thinking about it further and building on S Keith's comment, there are different phases in driving requiring different approaches: getting up to cruising speed and secondly maintaining a constant speed.

    The former, I suspect, requires engine speed at half peak-power (RedylC94), and then using the lowest revs for cruising. This translate into lower gears for getting up to speed and highest gear for cruising. This sort of fits with the minimum amount of accelerator I use when driving.

    I don't have a way to monitor fuel consumption but PaleMelanesian's data suggests highest gear at all stages of driving.

    Hmm, food for thought much appreciated, and happy Christmas to you all.
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  9. S Keith

    S Keith Well-Known Member

    Thanks for the clarification. Not the first time I've forgotten that the thermodynamics of a turbine engine aren't mirrored in an otto cycle engine.
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  10. ksstathead

    ksstathead Moderator

    Some general thoughts:
    1) with a stick, cruise at lowest RPM that doesn't lug under the terrain, etc. conditions.
    2) with an automatic, cruise at lowest RPM that doesn't lose TC lockup.
    3) on acceleration, one key often overlooked is to stop accelerating once a speed is achieved that avoids braking at the next light (don't accel to 40 psl if 35 lets you hold 35 right through the light but 40 has you braking).
    4) Moderate accel ok to use engine efficiency zone. No wheel slippage, of course, and avoid open loop operation that can come with WOT.
    5) Pulse & glide, subject to conditions, lets you save more fuel than steady state at times and also fit with other traffic better. So one might accel to 40 and glide through that light at 30 rather than cruise steady 35.
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  11. RedylC94

    RedylC94 Well-Known Member

    Yes. That plot shows the "typical" engine's peak efficiency is at roughly 40% of peak-power RPM, and at roughly 70% of maximum available torque at that speed. When driving for minimum fuel consumption, it's reasonable to aim at that ballpark when accelerating or climbing a long hill, but usually not when cruising on level stretches.

    The colors in that graph are cute, but not particularly enlightening, methinks!
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  12. phoebeisis

    phoebeisis Well-Known Member

    Optimal hp isn't where an engine is most efficient-in any sense-
    torque peak is probably where volumetric efficiency is "best" and maybe where hp per lb of fuel is best(because of the increased friction losses at the HP peak)

    1) having to accelerate " air" out of the way increases fuel consumption by square of increase in speed
    2) having to accelerate "oil" out of the way increases frictional losses-same story square of increase-not proportional
    double speed 2X quadruple fuel consumption increase because of aero load
    and engine speed frictional losses

    The tire to road surface power requirement-rolling losses-are proportional to speed
    average car-aero losses = rolling losses-about 55-60 mph
    tire pressure and tire choice are about all you can do about that-and keeping speed down

    so car speed kills-accelerating air
    engine speed kills-accelerating oil

    But "not locked up TORQUE CONVERTER" also kills
    so usually a speed like 47 mph -where you have locked up torque converter in top gear-
    is your best speed in a REAL WORLD CAR
    and BEST PRACTICAL SPEED ON THE HIGHWAY-is a bit higher- maybe 52-57 mph-depends on local conditions

    Now pumping losses- little bit different DIESEL vs Spark-
    but in general best practical speed is the same-where AT locks up 45 mph or so-but usually you have to drive a bit faster on the highway
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  13. phoebeisis

    phoebeisis Well-Known Member

    Yeah "car efficiency" as in miles traveled per unit of fuel
    is always going to be at a lower RPM than any measure of ENGINE EFFICIENCY

    UNLESS you put a tiny engine -not too many cylinders engine-in a large bulky vehicle-
    so perhaps a tiny 3-cylinder engine-
    but it has to have a LOT of flywheel effect -and ports intake runners NOT TOO BIG so you get plenty of intake velocity speed
    maybe making 20 hp at 2000 rpms and 30 mph

    Ford has in effect tried JUST THIS-sorta- with their little turbo 2.8 and 3.5 V6s in the 1/2 tons-
    but those engines are too BIG and too complicated
    and they spin too high-
    they don't BEAT the plain jane 3.6 Dodge
    and they don't beat the plain JANE 5.3 GM(maybe 1400 rpms at 60 mph)
    in real world or even EPA

    Yeah the small engine in big vehicle has been tried many times-(so it is operating closer to its HP PER UNIT OF FUEL PEAK- )
    But it doesn't work in USA because we always use BIGGER than needed engines and bigger port cross section than needed
    and more upper RPM power than optimal
    because we want ACCELERATION and peak speed(almost any USA car can do 100 mph-maybe could do 150mph if they weren't governed)
    Imagine the top speed -ungoverned-of a V6 ACCORD-probably 150 mph

    Same story many 1/2 tons-bet most could do 105 mph-some could do 130mph(ungoverned)

    so in the USA-big and slow turning-with plenty of inherent flywheel effect or plenty of "pulses" V8-to prevent LUGGING at LOW RPMS still "beat" or tie the little engined FORDS-and they are simpler cheaper
    Their throttle plates can be almost completely open at "pretty low rpms" but they don't lug-
    and the low RPS mean lower friction losses
    and their weight-with aluminum blocks-no fancy Turbo plumbing- not too big a deal-

    Now the Turbo Diesels-different animal- super high compression-love boost(spark motors don't like boost-detonation-so you really aren't getting the benefit of that almost free cylinder cramming) no throttle plate
    1) super high compression
    2) LOVE BOOST-see #1 allowing LOW RPMS
    3)no throttle plate
    4)probably have more inherent flywheel effect with heavier pistons rods cranks so love lower rpms
  14. phoebeisis

    phoebeisis Well-Known Member

    WOW- your little FIT doesn't lug at 26 mph in 5th gear?
    How many RPMs is that-maybe 1200 or so ?
    I wonder how Honda managed to prevent lugging-not much load of course-maybe 10 hp to do 26 mph
    Maybe Honda should give FORD some lessons
  15. NeilBlanchard

    NeilBlanchard Well-Known Member

    The lowest RPM that has enough torque, basically.
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  16. Die2self

    Die2self Saving more by using less!

    the trick it to be light with the go pedal, but yeah, I have had mine in 5th gear at 20 or so with out it lugging. It is not very responsive at all there but will accelerate slowly. :p
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  17. RedylC94

    RedylC94 Well-Known Member

    The engine of your Prius not-uncommonly runs fairly highly loaded at that speed or less. It does that because it's programmed to operate at the most efficient speed-torque combination that will provide the power level you're requesting. Is it "lugging"? The engine of my '81 Mazda (about the same size as that of a Fit, and older tech) would happily cruise at 1200 RPM in 5th (about 31 mph), as long as it didn't encounter a steep hill. I don't know whether it was lugging.
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  18. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    "Lugging" is a relative term. I worked with a German in the early 80's who had one of the first 5-speed Golfs (Rabbit) sold here. He told me that you need to be going 55 MPH before shifting into fifth or the engine would "lug". I had my GLC at that time , and I knew that was a crock , so I had a little (silent) laugh.
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  19. phoebeisis

    phoebeisis Well-Known Member

    Yeah "Lugging" is an imprecise term.
    I can "feel" it-I think

    I have always thought the Prius could get away with the relatively low RPMS and high loads
    Because it used its ELECTRIC MOTORS to in effect-prevent lugging
    When Lugging-lets call it the feel-a-ble decrease in crank speed in between pulses
    Yeah how in the heck could a human feel that-but we can-it is that slight "jerking"
    Anyway the Prius gets away with high load low rpms because as soon as it "detects" the slight decrease in crank speed
    it turns on the INSTANT TORQUE of the electric motors
    and instantly the crank speed smooths out
    Yeah the Prius engineers turned what should have been a disadvantage
    -Having to put torque into the wheels and the electrical system-
    which SHOULD have decreased efficiency because of -mechanical-electrical-mechanical

    But the Prius engineers turned it into a PLUS
    by allowing lower average RPMS than a pure gasoline motor driven car could get away with (unless it had a super heaver flywheel effect)
    Yes I know that some non hybrid gas cars can match the Prius in steady 60 mph FE-but not many-and not sure any 3000 LBS 175" ones can-even now-and non can beat it in city mpg(not even the TDs-which are more complicated more expensive)
    Prius engineers-very clever bunch-

    Must admit-I don't know HOW they detect lugging-seems a "continuous sensor or many many sensors" on the flywheel or crank would be needed-

    and I think the problem with LUGGING-is usually described as squeezing the oil out of bearings because of the extra time on power stroke
    Yeah I can't say that was ever a convincing explanation-
    but I don't have a better explanation

    Prius is a clever piece of engineering
    If all we wanted was an efficient relaible comfortable(front seat) cost effective transportation
    No one would buy ANYTHING but a PRIUS
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  20. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    It's mighty hard to beat the Prius , especially for city/suburban driving. My "normal" acceleration( not merging onto highway) is at 1500 RPM. Can you feel and hear the engine ? Yeah. Does it seem to be lugging ? No. When accelerating to highway speeds , I use a whopping 2000 RPM. It helps that we have nice long modern ramps around here.
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