Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Automotive Why engine efficiency drops for light loads?

  1. Mar 1, 2017 #1
    It is known that automotive engine efficiency (ie. fuel efficiency) is maximum at a specific range of medium loads. If we go higher than this range, the efficiency drops. If we go lower than these loads, the efficiency drops as well.

    Do we know the reasons for that? I am particularly interested in the low load efficiency decrease, but high load would be interesting as well.

  2. jcsd
  3. Mar 1, 2017 #2


    User Avatar
    Science Advisor
    Gold Member

    Well, at zero load the efficiency is zero by definition.

    If the curve of efficiency versus load is continuous and smooth, it must turn toward the zero-zero point. I don't need to know anything about engines to say that.
  4. Mar 1, 2017 #3


    User Avatar

    Staff: Mentor

    The "why" is at least partly a matter of friction being constant at a particular rpm. So if friction loss is 20% of engine output at 100% power, it is 50% of engine output at 40% power (and as said previously, 100% at 20%).

    My car burns about 30% as much gas at idle as it does at 60mph.
  5. Mar 8, 2017 #4
    Friction is not a constant. The friction increases with torque output due to side-loading on pistons and loads on bearings.

    However, there are losses that correlate inversely with torque output. Pumping losses, for example, are not as present with higher throttle positions and the engine's corresponding torque output. When the throttle is closed, the engine has to do work to decompress the intake charge inside of the intake tract downstream of the throttle. Inhale through a straw vs your mouth wide open. It takes more work to draw a smaller amount of air.

    Also, the dynamic and effective compression ratios change with RPM and Throttle position. When you draw a larger amounts of fuel and air in, the piston compresses it to higher pressures just prior to ignition (more mass in a fixed volume combustion chamber). More heat is released in a shorter amount of time, and a smaller portion of that heat is lost to transferring into the combustion chamber/cylinder/block before it is converted into torque.

    Most engines make their peak BSFC (Brake-Specific-Fuel-Consumption) while 80% loaded at the RPM where the engine would make peak torque if it were at 100% throttle/load.
  6. Mar 9, 2017 #5


    User Avatar
    Science Advisor

    Per hour or per mile?
  7. Mar 15, 2017 #6
    Congratulations to the first three answers, you missed the obvious, easy answer.. Its simple, it requires a calculable amount of horse power/torque to propel a vehicle of a certain weight and air resistance down the road at 50 mph or 60 mph or speed of the factories choosing.. The engine for said vehicle is designed to travel at the speed desired is designed with that requirement in mind.. You go faster it takes more power, more fuel, you go slower you drop below the efficient torque curve of the engine.. More fuel is required going slow because you are operating below the efficient torque curve so you have to give more fuel to make the required power in the inefficient portion of the power/torque curve of the engine...
  8. Mar 15, 2017 #7


    User Avatar
    Science Advisor

    If only the question was better worded and the answer could be so simple.
  9. Mar 15, 2017 #8
    Actually, you just gave an incorrect answer to a question that was not asked. Automobile engines do not achieve peak thermal efficiency at 50 or 60 mph. Do you know what brake specific fuel consumption is? The amount of work done per unit of fuel energy administered to the engine peaks at high loads. Generally 80-90% load. The reason cars don't get better gas mileage at those high loads is because the parasitic losses on the chassis increase exponentially with speed. Your engine might be doing more work per liter of fuel take in, but it has to do a disproportionately larger amount of work to overcome the wind resistance at those higher speeds.

    Engines have high brake-specific fuel consumption at low loads for the reasons I shared in post #4.

    Automobiles, as a system, get good fuel mileage between 50-70 mph because that is the fastest speed they go before the aerodynamics cause too much load on the engine. The engine runs more efficiently in terms of power output per unit of fuel, but the system achieves lower fuel economy because it is doing work that is not considered productive (accelerating air around and in front of it).

    This question, however, was about engines. The efficiency of one component of the system that is the vehicle. Not the overall system that is the vehicle. If engines were designed solely to give the vehicle peak fuel economy at a certain speed, they would be very small engines making around 30hp and cruising at nearly WOT. An understanding of BSFC will help you understand why that is the case.
    Last edited: Mar 15, 2017
  10. Mar 16, 2017 #9


    User Avatar
    Gold Member

    One factor is that there is time at slow speeds for heat to be lost from the charge (or air for a Diesel) as it is compressed, and from the hot gas as it expands.
    For a Diesel we do not usually have a throttle, so do not have pumping losses, which is an advantage at low speeds. There are also losses caused by valve overlap, which will be working in conjunction with an inlet and exhaust tract designed for a fairly high speed. Another is that if using a turbo charger, it is ineffective at light loads, so the engine drops back to being naturally aspirated.
  11. Mar 17, 2017 #10
    RogueOne... Com'on guy, you're picking fly poop out of pepper... Of course the design parameters for an engine to go into a particular type of automobile are far more extensive than what I described... I was mearly attempting to give a non-tech answer to the question... Didn't intend to ruffle your feathers and make you dig out all the big, impressive words from your text book...
  12. Mar 17, 2017 #11

    Randy Beikmann

    User Avatar
    Gold Member

    Actually, RogueOne was right to point out the correct answer in precise physics terms, and to avoid combining the "part load" factor as posed by the OP, with the "low vehicle speed" factor. His explanation was thorough, and avoided any ambiguity. I would take the opportunity to look up any terms he used that you don't understand.
  13. Mar 20, 2017 #12
    Not going to argue yours or his points for a second.. I was only trying to give a non technical answer to a simply stated question.. I can go farther in to detail of design and operations of the IC engine, as I'm sure both of you gentlemen are able, I just didn't see where the question warranted the lengthy, detailed answer..
  14. Mar 20, 2017 #13
    I'll entertain this off-topic post because I am sorry if I come across as rude. I want to say that my focus is to make my posts information dense. The goal itself is not to post jargon or one-up other members. Those words are in common use within this industry. Also, technically detailed answers are always welcome here. This is an engineering forum on a website called "Physics Forums" :wink: . I did not mean to belittle your post or your information. Accurate high-level answers are good, but I think they should be used as introductory paragraphs followed up by precise articulation of the factors and phenomena in the equation.
  15. Mar 21, 2017 #14
    RogueOne... Not to worry, my skin is pretty thick, I don't offend easily.. Nor was it my intention to slight you in any way.. We both obviously have a different approach when we answer questions from people.. Somewhere in my collection of files I hold a couple pieces of paper from an institutional of higher education that says I am one of those engineer guys, couldn't stand working in an office, I have to get my hands dirty.. I have found after 50 years, or so, of answering questions from co-workers and supervision that I'm best off not making my answer any more technical than the question.. If the person asking wants to dive in deeper, by all means I will.. It's just my way of doing things, made the mistake of making my supervisor, and his supervisor, feel like they didn't know what they were doing on some major electrical distribution for a job I was working.. Took those two guys almost 18 months to get past that incident.. Mad my life miserable for a while.. All due respect to you, Ill reserve myself to reading and keeping my comment to myself.... .
  16. Mar 30, 2017 #15
    One thing I don't think any one has mentioned yet.
    Engines run more efficiently with more compression, but low throttle has very little.
    I run a line from my crankcase to pull blow by and another to my exhaust.
    The exhaust line raises the compression with a neutral gas
    The RPM goes up just from adding pure exhaust then I drop it back down with the idle screw.
    Saves a bit of gas
  17. Mar 30, 2017 #16
    Powertrain matching:

    http://r.search.yahoo.com/_ylt=A0LEVyt8rt1YPnAAxipXNyoA;_ylu=X3oDMTEyZXNqZ25rBGNvbG8DYmYxBHBvcwMyBHZ0aWQDQjM2MjRfMQRzZWMDc3I-/RV=2/RE=1490951932/RO=10/RU=http%3a%2f%2fwww.sae.org%2fstudents%2fpresentations%2fpowertrain_matching_by_john_bucknell.pdf/RK=0/RS=gkamsANAdEAnLjIepj_fx_keHoo- [Broken]
    Last edited by a moderator: May 8, 2017
  18. Mar 31, 2017 #17


    User Avatar

    Staff: Mentor

    Sorry, no - the compression ratio of a piston engine is fixed due to the geometry of the piston/cylinder.
  19. Mar 31, 2017 #18

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    Thought experiment by a Non-mechanical engineer:
    If i wanted to make a machine that only wastes energy -

    i could build a cascade of two pumps
    that draws air into a chamber at maybe 1/4 atmosphere through a partially open valve (that's throttling)
    then pumps it back up to one atmosphere and exhausts it.

    That's what a lightly loaded gas engine does.
    As you approach WOT there's no throttling loss so that contribution to inefficiency disappears .

    I've always thought that was part of a Diesel's inherent advantage, it has no throttle plate so no intake throttling loss at part load.
    Am i correct that throttling process is irreversible?
  20. Mar 31, 2017 #19
    Really?! Wow...no
    are you not thinking?
    what is in the cylinder?
    How much compression do you get if there is no air to compress
    Only the maximum capacity of the cylinder is fixed
  21. Mar 31, 2017 #20


    User Avatar

    Staff: Mentor

    A couple of things:
    1. "Compression ratio" is not a pressure ratio, it is a volume ratio. Specifically, the ratio of the starting to the ending volume of the combustion chamber. Or to put it another way: "Compression" is the act of changing volume, not the act of changing pressure.
    2. Even if it was a pressure ratio, it would still be all about what is happeing in the combustion chamber, not the ambient pressure outside of the engine.


    If what you said were true, you'd be able to find a range or graph of compression ratios for engines instead of the fixed specification that it is.

    Now that said, there is an energy loss associated with air trying to force its way - unsuccessfully - through the intake when it is throttled down. This would manifest as a drag on the cylinder's movement. On the other hand, the exhaust has an easier time getting out of the engine.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: Why engine efficiency drops for light loads?