engine braking

There's also a bloody great throttling effect which stops the air going into the engine.

Correct me if I'm wrong, but also, centrifugal advance is just that, isn't it? i.e: it can only *advance* the ignition over it's static setting as speed increases and the weights fly outwards?

IIRC most vacuum advance units only advance timing too (but I have seen ones that do both).

You'll have to explain that one to me. In the extreme case, no air enters the engine. I'm not convinced that a lot of energy is expended on expanding and compressing the small amount of air remaining inside it.

Thanks GB,

I was simply extracting the **** out of young critcher on how a four stroke works rather than commenting on engine braking etc - and the last car that I worked on was about seven years ago was on my daughter's Rover 216 petrol injection to replace a broken cam belt and associated head-bits. Ill health and arthritis has stopped me since then, so I'm rather out of practice now and I let others do the work for me now - all good, gentle fun though.

Snipped

Damn, I could have phrased that a bit better. LOL

It takes a lot of energy to suck over 20 inHg on closed throttle with reasonable rpm.

None of which applies to a Diesel engine, they run wide open and have no braking. The compression energy is recovered by expansion on the down stroke, the piston "bounces" on an air spring. Diesel trucks can have a Jacobs "Jake" brake which releases the compressed air at TDC so the energy used compressing the air is wasted by just venting it. Then it pulls a vacuum on the down stroke.

critcher spoke with forked tongue...

So what's the problem with it?

If you have a problem with it, tell us the mechanism behind engine retardation.

"Chris Whelan" wrote

Not this load of nonsense again. Compression is not a braking mechanism.

The ignition timing advance/retard by centrifugal bob weights is entirely due to speed. Doesn't matter what the load is. As engine speeds up the weights move out restrained by springs, ignition advances. At some moderate rpm the weights contact stops and then the timing is fully advanced. As engine slows down the weights are pulled back by springs and ignition retards.

Vacuum advance/retard is load dependent. At high load / low vacuum the ignition is retarded. At low load / high vacuum such as idling the timing is advanced. As to defining advance or retard it's a matter of how the timing is set. Some were set at the advanced setting with idle vacuum connected so could only retard. While others needed the vacuum tube removed, so set at the retarded timing and could only advance.

Most cars with distributors had both speed advance using bob weights and vacuum advance/retard. Most motorbikes only had speed advance - it being taken that the throttle was an on/off switch used either at WOT full load or shut and no load timing didn't matter too much. Most 2 strokes had fixed timing.

Modern EFI engines have a 3D "map", rpm and load from AFM or MAP+temp are both used to look up the required ignition timing.

Have you ever tried to turn an engine by hand? Like with a starting handle? If you had, you might realise just how much compression matters. And modern engines usually cut off the fuel supply entirely on the over-run.

Driving instructors find that brakes are and always have been much cheaper to replace than clutch or gearbox synchro. You don't have to take the gearbox out to change brakes. Why encourage people to make lots of gear changes which they can botch expensively or fail the test though when they can just brake?

It takes a lot of power to accelerate fast. 0-60mph in 3 secs needs over

Years ago before the advent and common adoption of vented disc brakes, proportional servos, hydraulic brakes and MOT (1960 [1]) there was always a big risk of brake failure. Brake shoe lining would overheat on long steep descents, the pedal would hit the floor and all braking would be lost. Lining could and did de-bond or rip off the rivets. Ingress of water rendered drum brakes useless as unlike discs they didn't shed water but used it as a lubricant. The self servo effect of leading brake shoe wasn't very good, it could grab and wasn't easy to modulate. Cables could stretch and snap. The fact that modern cars are having "emergency brake assist" fitted shows that many people are just too lame to press the pedal as required even with a servo, they wouldn't have managed very well back in the 1950's [2]. Being in a low gear at the top of the descent meant in the event of brake failure the engine oversped and provided a lot of braking thus reducing the speed of the impending accident. People would sometimes live to tell the tale. If a high gear had been engaged the run away terminal speed would be just that. Depressing the clutch to select a lower gear only resulted in the runaway running away freely and too fast for the clutch to bite with the lower gear.

It would have been tantamount to murder for a driving instructor to teach dependance on the brakes.

For economy with fuel injections systems during long descents you have to keep the rpm above the fuel cut speed. That means selecting a lower gear. This is even more true for autos which will happily run the engine down to idle, burning fuel unless the O/D is off or 2nd selected.

Yes

Yes, it matters on the up stroke... But I haven't forgotten my O' level physics.

Yes. So when a modern engine is less able to slow a car down than an older engine, despite the older engine getting a contribution from a fuel supply, it shows a modern engine takes less energy to turn.

En el artículo , Chris Whelan escribió:

The modern mantra seems to be "brakes to slow, gears to go"

Some of my passengers look at me askance if I rev up in 3rd then change up to 5th, for example when joining a motorway. I don't see anything wrong with that, does anyone else?

It's something I do too sometimes. Another time I might block change from

Chris

So when I take the plugs out of an engine in order to make it easier to line up the cam timing marks, it's a figment of my imagination that makes it possible then?

Chris

Well, it's not really definable I guess. Perhaps pre-ECU?

Here are some random example CR's from Carfolio:

Of course, there will be some really old engines with high CR's, and possibly newer ones with low CR's, but overall the trend is higher.

Chris

Pumping air past a restriction is lossy, it's one of the main economy advantages of a diesel.