engine braking

...which has been going on for over twenty years now...

The concept that you need to 'balance the wear on the synchro', whatever that might mean. Oh, and the advice to select a gear that you are not actually going to use. I think that's called coasting...

1) Pumping effect. Higher in modern engines due to fuel shutoff on over- run, and higher CR. 2) Friction. Modern engines have more bearing surfaces than older ones; 5- bearing cranks, complex valve operating mechanisms, balancers on some engine, all add up. 3) Ancillary equipment. High output alternators, PAS, A/C, etc. (Some cars use information from the ECU to only put the alternator on load on the over-run (when battery state permits); the change in engine braking when the alternator loads up is discernible.)

Chris

Driving instructors are teaching the incompetent. We're talking - I assume - about the competent here.

Yep.

They are just what I picked. Looking at the 2012 Fiesta 1.2 and 1.4 engines, they are also 11:1 if you wanted a more recent example.

Chris

Oh, c'mon...

Balancer shafts were invented by Dr Fred Lanchester back in the 1920s, and made a come-back in the '80s.

Five bearing cranks (in straight fours) have been around since at least the '60s.

"Complex valve operating mechanisms" - by which I presume you mean variable valve, although again we're looking back 20 years - don't have any impact on the mechanical load. Some "complex valve operating mechanisms" explicitly reduce it.

Many truly _recent_ engine developments are intended to reduce mechanical load - computer-controlled alternators/water pumps/oil pumps which run light when required. The long-term reliability effect of that is yet to blow up in their faces... Then there's electric power steering pumps (although I'd query those as "recent" now).

Exactly what kind of timeframe are we talking about with "modern" and "recent"? I'd have said that the main emphasis on the most recent generation of vehicle designs had been to reduce mechanical load as far as possible, to reduce emissions.

The previous generation was increasing computerisation and impact protection. The generation before that was aerodynamic improvements. The generation before that was rustproofing improvements. The generation before that was FWD, packaging and suspension improvements. The generation before that was structural design - and we're back to the '50s/60s. Sure, there's always been cars "ahead of their time", I'm thinking in terms of the bulk of the market.

[...]

The title of the thread is 'engine braking' which surely implies on the over-run - the very time that these types of alternators are on-load...

Chris

Which, in itself, kinda disproves your point, since there's clearly no change in the last decade and a half... (About twice the average life expectancy of a new vehicle)

Heck, our VW's 10:1, and that's a mid-80s van...

That is the engine acting as a pump. Nowhere have I suggested that the act of pumping is free.

In a practical world there is heat exchange with the surrounding metal and an additional pumping contribution due to air leakage past valves & pistons. The pure act of compression and expansion is lossless.

The effect you sense of turning an engine by hand is not the same as what happens when it is operating.

Indeed, the engine is acting as a air pump. As I said earlier. So what is this "nonsense" to which Chris refers.

I think we might have different ideas of 'old' and 'new'!

In the context of this thread, drivers were taught to use engine braking back in the days when cars had mechanically operated drum brakes; engine braking was an essential aid.

The technique continued to be taught through the Sixties; brakes were hydraulic, and reasonably efficient, so it was arguably not necessary to teach engine braking.

Anything made in the last 20 years will have brakes that work very well. Not only is it unnecessary to use engine braking, modern traffic conditions make it undesirable.

I concur that there will be little change in average CR's since ECU's became common.

I could probably trawl Carfolio, and fine something from the '80's that had an atypically low CR, but it's not really the point I was making.

For the reasons I've outlined elsewhere, I believe over-run braking is higher with cars made in the last 20 years than it was with those of the Sixties and Seventies.

I could turn a 60's Mini engine on the bench against compression (with the plugs in) just using the crank pulley. (And I'm the original 7-stone weakling!) I doubt that many folk would do that with more recent engines.

Chris

[...]

WRT the effects of compression, why?

Chris

That's all very well for driving school dinky toys doing circuits round the test centre, but in the real world, anybody who's driven for instance a 7.5 tonner, even smaller vehicles, in hilly areas will know what a load of cobblers that is, especially if they're paying for their own brake pads/shoes.

To check out engine braking, put your car in 1st gear and come down the ramp from your multi-storey car park and take your feet off everything, you'll see.

Steve

Clearly. And that's precisely what I'm trying to get at.

I think your perception might be skewed here, due to you not quite appreciating which point of the age bell-curve you've reached...

I'm on the borderline where I can't even call myself "early 40s" any more. I learnt to drive in the late '80s. Anybody who learnt to drive in the '60s is either at or rapidly approaching retirement age now. The Sixties were half a century ago. They are ancient history. People learning to drive today were born in the mid Nineties.

And I'm suggesting that, in the context of changing advice over the last decade or two, due to "recent" car developments, that's an utter irrelevance.

[...]

I think you are missing the point about my contribution to this thread. I'm clearly of an age where engine-braking was taught, but I've said that it's obviously now an irrelevance. However, my first post here was to contradict DavidR's assertion that:

"A modern engine provides so little retardation that it hardly makes a difference." (To engine braking.)

I believe that to be untrue.

In terms of changing how one should drive, absolutely. In terms of a thread about engine braking, regardless of whether one should or shouldn't use it as a technique, surely it's wholly relevant and gives some understanding of why the advice has changed?

Chris

Going from high gear to second gear requires the second gear synchro to double the speed of the clutch. Using an intermediate increases the speed in two steps of roughly 1.4.

What time interval do you think distinguishes making a gear change from coasting? It takes hardly any extra time to slot the lever through two steps.

We've covered CR elsewhere. Pumping effect is almost certainly lower in a modern engine due to better internal aerodynamics and piston sealing. For instance, back in the 60s/70s a lot of straighforward tuning work could be done by matching manifolds to ports, cleaning up rough cast surfaces and such.

Modern engines are better made and run on lower viscosity oil.

It is noticeable how far a modern car will roll when spun up in first gear and the accelerator released. The fact that a modern non-turbo engine has a higher headline torque figure than an older one of the same capacity is straighforward evidence that a modern engine consumes less energy in its operation. It is not possible to at once consume less energy and provide more braking effect.

It might be daytime, the battery is fully charged, electric PAS is pointing dead ahead and AC is off.

I expect it does. One of the ways of giving lip service to the CO2 test (*) which probably offers very little real world benefit.

(*) Start the test with a fully charged battery and the test can be completed without loading the alternator. In real conditions, there is only limited scope for useful kinetic/potential energy recovery.

It's the grounds on which you believe it to be untrue that I'm questioning. Your logic and argument is based on skewed perceptions of "recent".

In message , Mike Tomlinson writes

Not if it gives better acceleration up to motorway speed. The worst sin is to crawl up to 45/50 mph and then create havoc as you try to leave the slip road. I had a couple of mates who did that, and it scared the hell out of me.

When you turn an engine by hand, you only notice the peak load during the upstroke. When the engine is operating, it is turning over the complete cycle and you don't see the stored energy moving between the air spring and flywheel mass.