1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A question about diesel engines

  1. Mar 25, 2016 #1
    I have read ( excuse me but i don't remember the reference) that a diesel engine, contrary to an engine that works on the otto cycle, is a " steady torque" engine meaning that torque doesn't change with revolutions.

    Could someone explain me the reason? ( in case that this statement is true ).
    Thank you
  2. jcsd
  3. Mar 25, 2016 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I drive a diesel powered car. The engine torque certainly isn't constant. However the torque vs rpm curve is certainly flatter than petrol/gas powered cars I have had in the past. I'm not sure why.
  4. Mar 25, 2016 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    To some degree, both the diesel and the otto cycles will produce torque curves which are relatively "flat" over a particular range of RPM. The diesel torque curve is probably flatter than an otto cycle's because most diesel engines are not designed to rev as high, and therefore do not suffer from increasing losses in induction efficiency as RPMs increase, which leads to a reduction in torque output. Also, many diesels are supercharged or turbocharged to provide increased power output.


    Above is a set of power and torque curves for a common automotive diesel engine. Not all such diesel engines exhibit the awesomely flat torque curve between 1600 and 2800 RPM, but many do.

    The curves below show how an otto cycle compares to a diesel cycle:

  5. Mar 25, 2016 #4
    Thank you for your answer
  6. Mar 26, 2016 #5


    User Avatar
    Science Advisor

    There are fundamental differences between Diesel and Otto cycle engines in the times of fuel combustion and the expansion stroke.

    An ideal Otto cycle engine compresses a stoichiometric mix of fuel and air. Pre-ignition must be avoided before the spark ignites and burns all in an instant explosion at top dead centre. Expansion of combustion products then takes place over the power stroke.

    An ideal Diesel cycle engine has excess hot compressed air at the end of the compression stroke. Only then is fuel injected as a mist, to burn as it is injected over a considerable period of the power stroke. This maintains pressure on the piston during the expansion stroke.

    The advantage of the Diesel cycle over the Otto cycle is that fuel and air are not compressed together in the diesel until the burn, so a higher compression ratio and cheaper fuel is possible with the Diesel. By avoiding the possibility of pre-ignition, doubling the compression ratio is possible with the Diesel which raises the thermodynamic efficiency to 150% of the Otto cycle 100%. That has significant economic and environmental implications.

    Over the last 100 years, better fuel injection systems and turbochargers have made Diesel engine performance very similar to Otto cycle engines, while maintaining the Diesel cycle efficiency advantage.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted