Hi compression pistons

37chief · Jan 18, 2017

Does a motor with a high compression ratio cause more wear on the rod bearings? A question I have often wondered about. Stan

ptfarmer · Jan 19, 2017

No, as long as the rod bearings has a good supply of oil to use as cushion between the bearings, and crankshaft.

MarkB_MI · Jan 19, 2017

The greater the torque produced by an engine, the greater the load on the rod and crank bearings. So in theory any engine modification that increases torque, be it pistons, camshaft, carburetor, turbocharger or whatever, will increase wear on the bearings. But in practice it's unlikely to be noticeable. Oil type, viscosity and cleanliness are more likely to affect bearing wear.

JDEM · Jan 19, 2017

It absolutely puts more stress on them. Premature wear is a different issue and depends on how well built the engine is. Raising mechanical compression ratio (with pistons) has a similar effect as using a turbocharger that raises the "effective" compression ratio.

Spook · Jan 19, 2017

Dude, did you used be a engineer? Other than that, I agree completely!😎

buickanddeere · Jan 19, 2017

As long as IF there is no additional ping, knock or detonation. The primary wear on the rod and main bearings will continue to be direct contact during dry starts before a oil wedge is formed for the bearings to "surf" on.
Feedlot tractors that are only shut down for fueling and oil changes last more hours than the same tractor in start stop utility duty.

rrlund · Jan 19, 2017

A diesel has more compression than a gas engine. Does the bottom end of a diesel wear out quicker? Not being cocky,just something to ponder.

dr sportster · Jan 19, 2017

I believe there will be more force applied to the rods and crank from the higher compression but it is nothing that the oiling system can't handle.

David G · Jan 19, 2017

I totally agree

ptfarmer · Jan 19, 2017

Look at the way the oil holes are in the crankshaft. When the rod journal is going up on the compression (and exhaust) stroke the oil is directed to the top rod bearing so it get the oil where it's needed the most.

Russ from MN · Jan 19, 2017

I agree that starts cause most of the wear, so I wonder why more engined don't have an electric prelube pump? I watched a video once on the
startup of a large piston aircraft engine and they ran an electric oil pump before turning it over. When I worked in the taconite plant they
always ran an electric oil pump before rolling over a mill, large babbitt bearings, 2 ft wide and 4 ft in dia.

MarkB_MI · Jan 19, 2017

> A diesel has more compression than a gas engine. Does the bottom end of a diesel wear out quicker? Not being cocky,just something to ponder.

Can't say without knowing whether the diesel generates more torque than the gas version of the same engine. Some do, some don't.

What everyone seems to be missing is that the force on the rod bearing during the compression stroke is MUCH LESS than the force during the COMBUSTION stroke. (If this wasn't true, the engine wouldn't generate any torque!)

JDEM · Jan 19, 2017

Obviously the people that design engines think so. For example, compare a John Deere 1020 rated at 39 max horsepower, gas or diesel. Gas engine with 7.5 to 1 CR only has three main bearings and the rod-journals are 2.3". Diesel version with 16.3 to 1 CR has four main bearings and rod-journals are 2.75".

MarkB_MI · Jan 19, 2017

> Obviously the people that design engines think so. For example, compare a John Deere 1020 rated at 39 max horsepower, gas or diesel. Gas engine with 7.5 to 1 CR only has three main bearings and the rod-journals are 2.3". Diesel version with 16.3 to 1 CR has four main bearings and rod-journals are 2.75".

Diesel and gas engines are apples and oranges. The combustion pressure curve for a diesel engine will be quite different than for a similar gas engine. The bottom end of the motor has to be sized to handle the PEAK combustion pressure of each cylinder. If the bottom end is sized to handle the combustion force, the compression force will be no big deal.

Texasmark1 · Jan 20, 2017

If you are talking about a radial aircraft engine, the problem was oil settling in the lower cylinders and if started as such would blow the head off or something of the sort. Usually you saw the ground crew rotating the prop a few revs, and then the flight engineer would cut the spark for a few revs and then when he switched it on the engine would light off and this belch of blueish smoke would come billowing forward. I think I read somewhere that the B-17 carried 55 gallons of oil for use in each mission.

ptfarmer · Jan 20, 2017

(quoted from post at 06:42:17 01/20/17) If you are talking about a radial aircraft engine, the problem was oil settling in the lower cylinders and if started as such would blow the head off or something of the sort. Usually you saw the ground crew rotating the prop a few revs, and then the flight engineer would cut the spark for a few revs and then when he switched it on the engine would light off and this belch of blueish smoke would come billowing forward. I think I read somewhere that the B-17 carried 55 gallons of oil for use in each mission.

I think the ground crews rotated the engine backwards to get any oil out of the bottom cylinders. Pushing the oil out on the exhaust stroke rather than hydraulic locking the engine.

Hi compression pistons

37chief

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ptfarmer

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MarkB_MI

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JDEM

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Spook

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buickanddeere

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rrlund

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dr sportster

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David G

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ptfarmer

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Russ from MN

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MarkB_MI

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JDEM

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MarkB_MI

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Texasmark1

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ptfarmer

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