What's new
What's new

OT - Combustion Pressure in Modern Gas Engine?

Lots of useful anti friction bearing crank designs.

Hardly-Ableson V-twins, And Kohler K series opposed twins use an assortment of plain, ball and tapered rollers. Sometimes all in the same engine ;-)
 
Pretty much every gasoline two-stroke engine I've ever seen uses rolling element bearings. Most of them rev higher than a four-stroke of similar displacement. Most of them have higher output than a four-stroke of similar displacement. Properly cared for they seem to last just fine.

.
There is a bmep limit to the roller bearings, above certain cylinder pressures they brinnell the races and destroy the running surfaces. I know this for first hand fact as I've seen the dimpled dents on wrecked cranks out of funnybike engines running roller bearings in extreme environments, and the fastest engines in modern top fuel bike engines are all based around hydrodynamic bearings.
 
Just to further answer the OP's question (and clarify for other posts - talking production engines here, not one-off, limited life race engines):

Max Gas Pressure

Gasoline:
2-stroke: 35 to 80 bar [514.5 - 1176 psi)
4-stroke: Naturally aspirated - 60 to 105 bar [882 - 1543.5 psi] , Turbo - 90 to 135 bar [1323 - 1984.5 psi]

Diesel:
Naturally aspirated - 80 to 100 bar [1176 - 1470 psi]
Turbo - 140 to 240 bar [2058 - 3528 psi]

Combustion temperatures are up to 2000°C with exhaust gas temperatures of 600-900°C

Since this diverged to bearings also:
Lets say we have a naturally aspirated gasoline engine running 6000 RPM, 90 mm bore with a max combustion pressure of 75 bar. The load on each piston/pin/bearing is about 5 metric tons, 50 times per second.
Pressure increase rates for gasoline engines is in the range of 3 to 6 bar/CAD (Crank Angle Degree) [If knocking is present, rates of 30 bar/CAD can exist.]
 
Thanks again to Tonytn36!

Interesting his point about hte rapid rise in bearing load.This is where plain, hydrodynamic film bearings shine, because the oil film is built during the lightly loaded portion of the revolution, and then simply does not have time to get out of the way of the heavy pressure.

The two-stroke Detroit Diesels had an interesting design feature, I suppose since the load on wristpins never reverses (under pressure from the blower even on intake stroke): the wristpin bushings are grooved with many axial oil-grooves. Exactly what you would NOT want for devlopment of hydrodynamic oil film in continuously rotating bearing, but apparently it works. Does anyone know the design history of this engine...were the grooves added in response to trouble, or is this trick common in continuously loaded oscillating bearings?
 
Not half rotational speed as in 5000rpm vs 10000rpm. Half rotational speed as in every other revolution. Like the combustion strokes in a four cycle engine or the nasty phenomenon of half-speed whirl

Thanks. Mark.

So how is the crank surviving this lack of load bearing capability, for 1/2 the rotational events? Or am I again just mis-understanding terminology?

Always "just one more" concept to wrap one's head around!

smt
 
I was under the impression that two stroke gudgeon/wrist pin bearings were a special case where hydrodynamic bearings do not work well past a certain level of power output. This is not connected with the use of petroil lubrication or otherwise. I do not think any serious high powered two stroke motorcycle engines from the 60s onwards used plain bearings here. The lack of any thrust reversal is apparently part of the reason, but I don't understand why - it should be possible to build an oil wedge under these conditions.
 
Thanks. Mark.

So how is the crank surviving this lack of load bearing capability, for 1/2 the rotational events? Or am I again just mis-understanding terminology?

Always "just one more" concept to wrap one's head around!

smt

I don't know, because I'm not a tribologist, but the little I've been able to pick up on the interweb when looking for that very answer, without having to buy very expensive tribology books is that you have to make sure that the half per rev firing impulses don't dominate the forces on the bearing. Cylinder firing order makes a difference, V as opposed to inline engines makes a difference, the weight distribution of crank, conrod and piston all make a difference. Bearing clearance and shape also make a difference.

The impression I got was that it doesn't often cause bearings to get wiped, but has been known to in 'unlucky' designs. I guess you wouldn't really want to be the engine designer when the CEO comes to ask you about the recall program if your engine was the one with a problem...
 
The two-stroke Detroit Diesels had an interesting design feature, I suppose since the load on wristpins never reverses (under pressure from the blower even on intake stroke): the wristpin bushings are grooved with many axial oil-grooves. Exactly what you would NOT want for devlopment of hydrodynamic oil film in continuously rotating bearing, but apparently it works. Does anyone know the design history of this engine...were the grooves added in response to trouble, or is this trick common in continuously loaded oscillating bearings?

Wrist pins were a source of problems in the 2 stroke Detroits. Back in the '70's, I recall specific oils that were formulated for Detroits. They contained special additives for the wrist pin joint. More modern oil formulations provide sufficient lubrication without additional additives.

As for the evolution of the engine, its probably the longest running production engine out there. First marketed in the '30's and produced into at least the '90's, its lifespan can only be rivaled by the small block Chevy.

I don't think a wrist pin qualifies as a hydrodynamic bearing. Its a splash lube oscillating bearing.
 
It would be rather interesting to know what the combustion pressures are in a top fuel dragster or funny car producing something close to 10,000 horsepower. Whatever they get to is enough to trash most of the rotating assembly in just 4 or 5 seconds! the crank seems to survive, but the rods and pistons are replaced after every pass! I've been told that the cranks are hardened 4340, but I really wonder if even 4340 can take that kind of abuse.......

Frank
 
I don't know, because I'm not a tribologist, but the little I've been able to pick up on the interweb when looking for that very answer, without having to buy very expensive tribology books is that you have to make sure that the half per rev firing impulses don't dominate the forces on the bearing. Cylinder firing order makes a difference, V as opposed to inline engines makes a difference, the weight distribution of crank, conrod and piston all make a difference. Bearing clearance and shape also make a difference.

The impression I got was that it doesn't often cause bearings to get wiped, but has been known to in 'unlucky' designs. I guess you wouldn't really want to be the engine designer when the CEO comes to ask you about the recall program if your engine was the one with a problem...

The so called "Big Bang" engines must really stress a system a lot more than originally intended ?? "Big Bang" being all cylinders firing on one revolution of the crankshaft ?? Supposedly in some super bike stuff it gives you a sort of anti lock brakes in reverse, if there is a loss of traction while in a turn it is a sporadic loss not a constant loss, giving the rider more retained control ?? My dad told of some attempt to do a "Big Bang" on a V8 and whoever was trying it kept blowing the crank clear out the bottom of the block.

Bill
 








 
Back
Top