5

Basically, a car (street legal 2.5L) has about 7k RPM on its tachometer and a Motorcycle has about 14k RPM. Why is there a difference?

Is there any relation between the displacement of the engine and the weight of the vehicle?

Moreover, how can they both do the equal speed while their tachometers are showing different RPM's (considering they both are on their top gears)?

Is is something related to the gear ratios?

6

One important factor is the mass of the components. Smaller, lighter components and shorter strokes allow higher RPMs. Given equal RPMs, an engine with a bigger stroke will put higher stresses on the components, which means they need to be stronger, which means they need to be heavier, which increases the stress even more.

Motorcycle engines tend to have smaller and lighter parts than car engines, and while this limits their maximum throughput, it allows for higher RPMs, which allows for more power.

Cars weigh more, so they need more torque at lower RPMs, and a heavier engine is less of an issue since the whole vehicle already weighs a lot. Motorcycles are smaller and lighter, so getting more power from a smaller engine generally makes more sense than putting a big heavy one in (although that does not prevent people from doing so.)

It's fundamentally a tradeoff, and which is best depends on application. Big diesel engines have really long strokes and produce peak power at lower RPMs. Ship diesels have pistons the size of cars, and output their peak power at hundreds of RPMs instead of thousands.

RPMs vs street speed is of course dependent on gearing primarily, and even two cars probably don't have the same speed at the same RPMs.

  • "Big diesel engines ... produce peak power at lower RPMs." I seem to recall reading that large ship diesel engines commonly run in the several dozen revolutions per minute or so range. – a CVn Oct 21 '16 at 19:03
  • 1
    Yeah, 100-200 RPMs is common operating speed, and idle speeds can be much lower. THis guy idles at 22 RPM, which is pretty impressive considering the stroke is 2.5 meters. amusingplanet.com/2013/03/… – barbecue Oct 21 '16 at 19:08
  • At 102 RPM the lRT-flex96C puts out over 5.6M lb-ft of torque. – Pᴀᴜʟsᴛᴇʀ2 Oct 21 '16 at 19:15
  • Yes it is a real kick to be in the engine room of a big diesel ship. And even having low RPMs they still use reduction gears some have 30 or more ratios. – spicetraders Oct 21 '16 at 19:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.