Engine efficiency

[Ecky]

Originally Posted by Old Mechanic

Always a trade off in reciprocating engines. Longer rods and longer stroke generally give you more torque at lower RPM, but also increase reciprocating mass which will reduce efficiency.

I think the 225 Chrysler slant 6 had a 3.5 inch bore and a 4.5 inch stroke.

regards
Mech

After some reading, I'm guessing the reason the Slant 6 was a... well, Slant 6, was so they could make it extremely undersquare to develop a lot of torque at low RPM, efficiently, but also have long enough rods that the rod:stroke ratio wasn't too poor (and thus causing excess friction, and bending rods all the time), yet still fit under the hood?

[Diesel_Dave]

A longer rod also reduces the side forces of the piston so there's less friction between the rings and the cylinder walls.

[drmiller100]

Originally Posted by Ecky

After some reading, I'm guessing the reason the Slant 6 was a... well, Slant 6, was so they could make it extremely undersquare to develop a lot of torque at low RPM, efficiently, but also have long enough rods that the rod:stroke ratio wasn't too poor (and thus causing excess friction, and bending rods all the time), yet still fit under the hood?

yeah, there is kind of two conversations going on at once here.

there is bore to stroke ratio. perception has always been longer stroke means more power at lower rpm, bigger bore means you can fit bigger valves for higher RPM horsepower.

Also, there is stroke to rod length. after doing a bunch of reading, smokey yunich used to believe longer rods are "free" power, but more modern theory is there is a perfect ratio. too long of a rod can lead to issues also, but too short of a rod really sucks.

it seems to me retarding the exhaust timing leads to better bottom end power and better gas mileage.

[Diesel_Dave]

Originally Posted by drmiller100

Diesels do not have pumping losses (no throttle plate restriction).

That's not entirely true. You are correct that diesels typically don't have throttling losses, however, there still are pumping losses. This includes all the losses associated with getting the air & exhaust through the manifolds & ports. In addition, in a turbodiesel, there are turbo inefficienies that are considered pumping losses as well. In some cases, however, if the turbo is being run in an efficient manner (in the sweet spot of the turbo maps), there are some speeds and loads where the tubo actually results in "positive" pumping work (negative pumping losses). The intake manifold pressure can actually be higher than the exhaust manifold pressure. I imagine this might also be possible on a gasoline turbocharged engine, but that's not really my area.

[drmiller100]

interesting.
i did not know you could have higher intake pressures then exhaust pressures. thinking about it, theory says it sure is possible. though.

thank you.

[Old Mechanic]

One of my favorites, no reciprocation, no masses changing direction, kind of like a piston in cylinder turbine.

Animated Engines - Gnome Rotary

A true rotary engine. Even a centrifugal supercharger.

regards
Mech

[pete c]

Originally Posted by drmiller100

longer rod = more better. more efficient, more power, more everything, but slower maximum piston speed, which again, is better.

there is another reason a long rod helps. (insert obligatory tasteless joke here) it decreases the side load on the piston when the crank is at 3 and 9 o'clock. These side loads play hell on the piston meaning it has to be bigger with short stroke engines.

so, when power to weight ratio is concern number 1 as with a MC, sports car or aircraft, light compact short strokers are the way to go, but, they will be less efficient.

stationary engines should have very over square engines as weight is of no concern.

high efficiency vehicles will likely be something somewhere in the middle, probably a little oversquare, ideally.

[drmiller100]

Originally Posted by Old Mechanic

One of my favorites, no reciprocation, no masses changing direction, kind of like a piston in cylinder turbine.

Animated Engines - Gnome Rotary

A true rotary engine. Even a centrifugal supercharger.

regards
Mech

looks kind of like a hydraulic motor / pump.

[pete c]

Originally Posted by Old Mechanic

One of my favorites, no reciprocation, no masses changing direction, kind of like a piston in cylinder turbine.

Animated Engines - Gnome Rotary

A true rotary engine. Even a centrifugal supercharger.

regards
Mech

there still is some reciprocatin' going on as best I can tell. it sure is a cool engine to watch run though. radial engines really are works of art.

how do they fall in the efficiency area? is there a reason they are no longer made other than prohibitive production costs?

[drmiller100]

Originally Posted by pete c

there is another reason a long rod helps. (insert obligatory tasteless joke here) it decreases the side load on the piston when the crank is at 3 and 9 o'clock. These side loads play hell on the piston meaning it has to be bigger with short stroke engines.

so, when power to weight ratio is concern number 1 as with a MC, sports car or aircraft, light compact short strokers are the way to go, but, they will be less efficient.

stationary engines should have very over square engines as weight is of no concern.

high efficiency vehicles will likely be something somewhere in the middle, probably a little oversquare, ideally.

you confusing rod length and stroke. different things.