A question about detonation

[tail end charlie]

Very good post charlie.

Messy

I had a friend who spent a fortune tuning a CB750 honda,big carbs, high lift cams, bigger pistons, valves electronic ignition special con rods etc etc etc + he installed it in a rickman chassis. He then decided to get a turbo which he imported from the US (this was 1979) to fit the turbo he had to undo almost all the previous mods. By the time he got everything sorted he could have bought a bike out of the showroom with more power and reliability and possibly handling

 

[tail end charlie]

Fuel quality and compression ratios are a huge factor in preventing detonation. A octane rating is a measure of a fuels resistance to burning. That is why high compression engines require high octane fuel. Higher compression makes more heat. Typical pump gas will only support on average 10:1, or 10.5:1 compression safely. Anything over that and you can can have trouble with engines knocking, pinging, or run-on after you turn off the ignition. Our open modified dirt track engines will be in the 13-14:1 compression ratio range with over 600hp, and racers will typically run 110 racing fuel, or straight methanol. It is actually better to have too much fuel than too little as too much fuel will flood your engine, but typically not cause any sever problems. A lean engine with detonation problems can destroy a new engine in a few minutes if the detonation is severe enough. We see racers all the time who have trouble with detonation either do to poor fuel quality, not enough fuel, timing problems, etc burning up $700 sets of pistons. There are many reasons detonation can occur, TEC's explanation in the #3 post is a good explanation.

Messy

I raced two strokes which I ran on Avgas, with a two stroke the actual compression ratio is the mechanical compression measured from the volumes at BDC and TDC + the compression effects of crankcasethe ports the exhaust and the harmonics. I therefore used a an engine tuned for standard fuel and the avgas meant I wasnt rebuilding engines in a regular basis.

 

[Messy1]

I work for a dirt track business that sells engines, chassis, parts etc. We have a first rate machine shop known throughout the country in our field of racing, and we do all machine, assembly, research and dyno everything in house. We see the effects of detonation all the time, burnt up pistons, exploded motors, bent connecting rods. Never a good scenario, you have to have everything tuned perfectly, fuel, timing, etc.

 

[tail end charlie]

I work for a dirt track business that sells engines, chassis, parts etc. We have a first rate machine shop known throughout the country in our field of racing, and we do all machine, assembly, research and dyno everything in house. We see the effects of detonation all the time, burnt up pistons, exploded motors, bent connecting rods. Never a good scenario, you have to have everything tuned perfectly, fuel, timing, etc.

I had a friend whose motor siezed at 140MPH (TZ250max revs) on the Isle of Man poor guy was in hospital on the Island and back home for 2 yrs. It isnt just con rods that get bent with the wrong mixture!!!!!!!!!

 

[Messy1]

That's sounds bad. Sounds like your friend is lucky to be alive.

 

[tail end charlie]

That's sounds bad. Sounds like your friend is lucky to be alive.

Yeh he was lucky but he did set a 100MPH lap which was his long time ambition.
The Isle of Man is notorious (dont know if youve heard of it) but coming down the "mountain" the motor is at or near max revs for about 4 minutes, 1 three lap race is about 120 miles and the whole circuit is lined with dry stone walls, trees and lamposts. The year I went a competitor was killed when he hit a horse that was spooked by the noise and jumped into the track?????

Heres a flavour of it, the second video is "down the mountain"

View: https://www.youtube.com/watch?v=-Nzq2GorVH4

View: https://www.youtube.com/watch?v=scfiltMqHhc

 

[PWR4360-59B]

I guess technically that could be true, only if your engine has so little fuel in the mixture that it will not ignite during the combustion cycle. We see more engine problems and damage due to detonation from lack of fuel, timing, etc than any other problem. Reading spark plugs will tell you if there is a detonation problem. White electrodes are caused by a lean mixture. Wet, dark plugs by too much fuel

This is how the airlines operating the Turbocyclones got into the .32 BSFC area in the 50's, they called it SUPERLEANING. I don't remember the A/F ratio right off.

 

[Colin1]

This is how the airlines operating the Turbocyclones got into the .32 BSFC area in the 50's, they called it SUPERLEANING. I don't remember the A/F ratio right off.

Lindberg showed Pacific pilots how to autolean P-38s
but autoleaning it required dropping standard rpm by roughly 30% and slightly increasing manifold pressures

I don't know what the difference is between autoleaning and superleaning (if there is one) but perhaps it might follow a similar methodology otherwise I can't see how MTBO figures wouldn't take a hit

 

[Shortround6]

Super lean ratios do not have the tendancy to detonate because of the cooling effect of the excess air.
Aircraft engines tend to have ineffcient combustion chambers so that doesn't help either.

Gasoline is about 9000 times denser than air. It is much easier to put more gasoline into an engine than is needed for combustion for extra cooling than to try to use extra air.
Most engines don't need extra cooling when operating in cruise mode. If they do then something is really screwed up.
Since the limiting factor in producing maximum power is airflow through the engine running lean mixtures, even if they don't lead to detonation, is limiting peak power.
Designing for maximum power from a given size or weight of engine is rather different than designing for maximum fuel economy.

Engines are not controlling explosions, they are controlling rapid combustion. The burning of the fuel mixture in the cylinder, while very rapid, is planned and predictable. The spark plug/s initiate the combustion and the flame front/s spread out from the plug/s and travel across the cylinder through the space between the piston top and the cylinder head. Internal contours help control the spread and cooling. If the mixture is exploding somethng has gone wrong.

Detonations in supercharged engines is a bit different than detonation in un-supercharged engines or "pinging". In the worst case of a supercharged engine the fuel- air mixture is heated almost to it's auto ignition temperature by the supercharger and the compression in the cylinder. One thing going wrong (hotspot in cylinder) and the whole cylinder full of fuel-air ignites at once in a true explosion instead of the normal progressive burn. Given the large size of aircraft cylinders (single cylinder on an R-2800 is just over 2.5 liters) and their light weight construction (it wouldn't be until the 1960s-70s that race car engines achieved the same power to weight ratios of WW II aircraft engines) and detonation could get catastrophic in a hurry. Some aircooled engines were known to blow off cylinder heads or entire cylinders from the crankcase.

 

[tail end charlie]

In the worst case of a supercharged engine the fuel- air mixture is heated almost to it's auto ignition temperature by the supercharger and the compression in the cylinder. One thing going wrong (hotspot in cylinder) and the whole cylinder full of fuel-air ignites at once in a true explosion instead of the normal progressive burn. Given the large size of aircraft cylinders (single cylinder on an R-2800 is just over 2.5 liters) and their light weight construction (it wouldn't be until the 1960s-70s that race car engines achieved the same power to weight ratios of WW II aircraft engines) and detonation could get catastrophic in a hurry. Some aircooled engines were known to blow off cylinder heads or entire cylinders from the crankcase.

I was using general terms, what is an explosion in normal life is actually rapid combustion, no one ever says there was a rapid combustion of gas in a house.

What you describe above is still rapid combustion which because it is initiated at a hot spot will still travel in front. True detonation in an engine sense is where pockets of fuel air mixture outside the flame front reach a temperature and pressure sufficient that they explode. Instead of exerting a growing pressure on the piston there is a sudden shock which damages the engine. Repeated running of a knocking engine damages it as you say, the shocks can eventually cause catastrophic failure, I have seen a BSA 650 twin shear the barrels.

 

[PWR4360-59B]

Gasoline is about 9000 times denser than air. It is much easier to put more gasoline into an engine than is needed for combustion for extra cooling than to try to use extra air.

Yes but especially now, air is much less expensive than Gasoline.
And thanks to ADI the gasoline can be leaned on during max power jaunts.