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Engine Braking
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When changing down, particularly changing down several gears in one jump, you need to increase engine revs to match the lower gear or the clutch will slip and wear until the speeds match (or you may just stall the car).
What do you do then when you change up?
Slow down the car so that he gearbox speed will match the lower engine revs?
You match your speeds before you change gear ...i.e you make sure that you are going fast enough / slow enough for the gear that you want to select next so that the engine won't stall or over-rev.
Other than that you let the clutch in and out quickly without lingering on the pedal and that's that ...it's what it's designed for. No need to confuse yourself and fiddle with the accelerator.
Rev matching was necessary on non-syncromesh gearboxes, otherwise you couldn't engage gears ...but that's different from the workings of the clutchNo not for changing up definitely not!!
If driving at 40mph in 4th at 2,000rpm and want to change to 3rd then the revs will be higher for the same road speed. Suppose they would rise to 3,500rpm. Then keeping foot on acc and popping in the clutch will let the revs rise to 3,500rpm then engage the gear by letting out the clutch. Less wear and more comfort!
Why would you need to do that?
If you're comfortable at the speed you're going, stick in fourth ...if you want to slow down and use engine braking, why speed up the engine first and loose the braking effect?
The only time that rev matching makes sense is when you're changing down to speed up. And even then it's not to save the clutch, but to keep the momentum going.0 -
What do you do then when you change up?
Slow down the car so that he gearbox speed will match the lower engine revs?
You match your speeds before you change gear ...i.e you make sure that you are going fast enough / slow enough for the gear that you want to select next so that the engine won't stall or over-rev.
Other than that you let the clutch in and out quickly without lingering on the pedal and that's that ...it's what it's designed for. No need to confuse yourself and fiddle with the accelerator.
Rev matching was necessary on non-syncromesh gearboxes, otherwise you couldn't engage gears ...but that's different from the workings of the clutch
Why would you need to do that?
If you're comfortable at the speed you're going, stick in fourth ...if you want to slow down and use engine braking, why speed up the engine first and loose the braking effect?
By changing to 3rd you would increase the engine braking not decrease it. You dont have to do any of it of course you can simply change gear in the normal manner, but it is something interesting to try none the less.0 -
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Just about as mad as dumping the accelerating energy into the clutch when you step on the loud pedalThat is only kind of engine braking anyone would recommend!
What you are talking about is dropping the clutch when the engine speed is slower than required for a lower gear, and dumping the braking energy into the clutch. That is a mad idea.
Try to read what other people write. Engine braking does of course work going downhill ...but the brake force is reduced when speed up the engine first.Generations of truck drivers will be fascinated to learn that engine braking doesn't work. All those signs on the French motorways I passed this summer, telling trucks to use engine braking on 5% gradients, all pure fantasy!
In the case of a diesel engine there actually is a point where it will run away from you ...that's why trucks have motor brakes and/or retarders to assist with braking (btw ..I have driven trucks)
That just doesn't make any senseYou did: when you say that engine braking doesn't work if you match engine speed to the appropriate gear, you're talking about slowing the car using the clutch while the engine is spinning slower than required. The only way you can do that continuously for a 10 mile downhill stretch is by riding the clutch.
No, you're talking about slowing the car with the clutch, a completely seperate thing from engine braking, and a really bad idea.
I never said to slow the car with the clutch ..I don't know where you are getting that from0 -
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ok ...I'm willing to learn !
Here's me, nursing my fully laden 3.5 ton camper along a very twisty road, shifting up and down, using engine braking as well.
What am I doing wrong ? (other than nearly labouring the engine near the end there:D)
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ok ...I'm willing to learn !
Here's me, nursing my fully laden 3.5 ton camper along a very twisty road, shifting up and down, using engine braking as well.
What am I doing wrong ? (other than nearly labouring the engine near the end there:D)
Don't worry about them. The main question is how long does the clutch last in miles. If it's above the average, you're better than average with the clutch.
Thinking about it, I need to change a clutch on the AX. Oil contamination, and the coke trick's stopped working........:o0 -
I just want to add that I agree with Zube that peasant's "clutch braking" is a bad idea. I do it myself from time to time, but it's still a bad idea
A vehicle is obviously only undergoing "engine braking" if the engine is connected to the wheels (i.e. the clutch is fully engaged and there is no slippage) and it is a relatively effective to slow a vehicle. To examine its effect, put her in 1st and drive to the redline, then quickly and completely lift off the accelerator. Notice how your forehead bangs into your windshield?
Peasant's point that you can get additional braking force by scrubbing the clutch while bringing car and engine speed in line is valid, but it must be one of the worst things a reasonable person can do to a clutch. You may think that if you drop the clutch instantly and don't ride it, that you won't be getting any slip, but the whole time the engine is revving up to match the car's speed has to be accompanied by slippage.
One last point. Diesel engines do have significantly less ability to engine brake than petrol engines. I've heard this is because diesel engines leave the valves closed when the throttle's off, so you don't get any resistance from air being squeezed through them.
When signposts tell truck drivers to be in a low gear on a slope, they're probably suggesting that the truck driver also use (what's known over here as) a jake brake. I'm not sure exactly how they work, but I think they open the valves at the "wrong" time to cause lots and lots of resistance. They're also very loud
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When signposts tell truck drivers to be in a low gear on a slope, they're probably suggesting that the truck driver also use (what's known over here as) a jake brake. I'm not sure exactly how they work, but I think they open the valves at the "wrong" time to cause lots and lots of resistance. They're also very loud
http://en.wikipedia.org/wiki/Jake_brake
+1The idea of slowing down on the brakes to a point where you cannot slow down anymore without stalling before changing down is a bad idea.
As an owner of a car that cut out easily at low/idle revs (dirty idle control valve, probably fuel filter too as it still sometimes happens), I cannot see how this is good. If your engine goes, so does your power steering and hydraulic brakes - you're definately not in control if that happens!
If I need to stop but have a long distance to do so (e.g. junction off a dual carriageway leading to a crossing or roundabout, red traffic lights etc.) I would use engine braking. I'd rather wear my clutch a bit more than coast TBH.
Does the engine actually consume any fuel when engine braking?0 -
One last point. Diesel engines do have significantly less ability to engine brake than petrol engines. I've heard this is because diesel engines leave the valves closed when the throttle's off, so you don't get any resistance from air being squeezed through them.
When signposts tell truck drivers to be in a low gear on a slope, they're probably suggesting that the truck driver also use (what's known over here as) a jake brake. I'm not sure exactly how they work, but I think they open the valves at the "wrong" time to cause lots and lots of resistance. They're also very loud
Petrols have better engine braking because when you go of the gas you also close the throttle valve, effectively closing the air intake.
Both engines on braking compress air (which slows them down) but the compressed air also wants to expand again on the downward stroke and pushes the piston back down, so in effect you loose very little energy. But in the petrol engine the air intake is drastically minimised, so it has to pull a vacuum, which really hepls to slow it down. The diesel just keeps on puffing air in and out.
Additional engine brakes in diesels work in two ways:
- exhaust brakes; close a valve in the exhaust so that less air flows out and air accumulates, further increasing the resistance
- "jake breaks"; open the exhaust valves prematurely once the air is compressed, storing the air in a separate chamber, at the same time on the downward stroke the piston is not only not pushed down by the compressed air but also has to pull a vacuum. This increases engine braking signifcantly ...the action of blowing off the compressed air is also very noisy.0 -
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According to the bastion of truth that is wikipedia: In a gasoline engine, some engine braking is provided during closed-throttle operation due to the work required to maintain intake manifold vacuum, the balance coming from internal friction of the engine itself. Diesel engines, however, are unthrottled and hence do not provide engine braking from throttling losses. (from http://en.wikipedia.org/wiki/Jake_brake)My 'real world' experience of engine braking is mostly in off-road driving, particularly when descending steel hills. I've always found diesels to be much more effective at slowing/controlling vehicle speed at that lark than petrol engines.
That's just my experience of them.
Okay, I misunderstood, but the Jake Brake still works by opening a valve at the "wrong" timeI've never heard of an engine that can do this. 0 -
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