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Engine Braking

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Comments

  • #52
    peasant
    Registered Users, Registered Users 2 Posts: 17,819 ✭✭✭✭peasant


    I guess we're both "right"

    Yes, I do (ab)use the clutch to a degree for engine braking ...because if I matched revs I wouldn't have very much braking left on my diesels.

    But I do match the speed before I change down with a quick touch of the brakes, so the wear on the clutch isn't as bad as it sounds, (I'm yet to wear out a clutch on any vehicle prematurely.)


    Hard to explain ...you would have to be there really


  • #53
    peasant
    Registered Users, Registered Users 2 Posts: 17,819 ✭✭✭✭peasant


    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)

    https://www.youtube.com/watch?v=R2aIaxxK_Bk


  • #54
    ianobrien
    Registered Users, Registered Users 2 Posts: 2,755 ✭✭✭ianobrien


    peasant wrote: »
    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)

    https://www.youtube.com/watch?v=R2aIaxxK_Bk

    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........:o


  • #55
    Balfa
    Registered Users, Registered Users 2 Posts: 1,709 ✭✭✭Balfa


    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? :D

    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 :|


  • #56
    zilog_jones
    Registered Users, Registered Users 2 Posts: 6,638 ✭✭✭zilog_jones


    Balfa wrote: »
    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
    The 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.
    +1
    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?


  • #57
    peasant
    Registered Users, Registered Users 2 Posts: 17,819 ✭✭✭✭peasant


    Balfa wrote: »
    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.


  • #58
    Mr.David
    Registered Users, Registered Users 2 Posts: 1,703 ✭✭✭Mr.David


    Pete67 wrote: »
    Lots of different views here. I'll add mine, from an engineers perspective. Engine braking is a useful tool to control vehicle speed, particularly on downhill gradients. However it should not be used to reduce vehicle speed suddenly by transferring the kinetic energy of the vehicle into increased engine rpm. This will cause additional wear of the clutch friction plate as it must slip until the two speeds are matched. The footbrake should be used for this purpose - it's far cheaper and easier to replace brake pads than to change out a clutch. If the driver changes down matching engine speed to road speed for the new gear before engaging the clutch then there is no slip and hence no wear on the clutch. In fact, the only time a skillful driver will slip the clutch is when starting the vehicle from rest at the lowest engine speed possible to provide enough engine torque to get moving without stalling.


    Well put.


  • #59
    Rovi
    Closed Accounts Posts: 8,570 ✭✭✭Rovi


    Balfa wrote: »
    Diesel engines do have significantly less ability to engine brake than petrol engines.
    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.
    Balfa wrote: »
    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.
    I've never heard of an engine that can do this.


  • #60
    Balfa
    Registered Users, Registered Users 2 Posts: 1,709 ✭✭✭Balfa


    Rovi wrote: »
    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.
    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)
    I've never heard of an engine that can do this.
    Okay, I misunderstood, but the Jake Brake still works by opening a valve at the "wrong" time :)


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  • #61
    GreeBo
    Registered Users, Registered Users 2 Posts: 27,253 ✭✭✭✭GreeBo


    ianobrien wrote: »
    If you want to see an almera sideways, pm me and we'll meet up and I'll show you how it's done
    You're my hero! :pac:
    If you are sliding then you have inappropriate speed for the road conditions.

    I went through a phase of engine breaking but Ive stopped as I found I had to concentrate on it.
    Now I shift up once I reach 2000rpm and down as appropriate.


  • #62
    peasant
    Registered Users, Registered Users 2 Posts: 17,819 ✭✭✭✭peasant


    GreeBo wrote: »
    You're my hero! :pac:
    If you are sliding then you have inappropriate speed for the road conditions.

    What's that got to do with engine braking?

    Just keep it on topic please


  • #63
    Zube
    Closed Accounts Posts: 1,502 ✭✭✭Zube


    peasant wrote: »
    What's that got to do with engine braking?

    We've had normal braking, engine braking and clutch braking, I think the lads were branching out into rear three-quarter panel braking, passenger door braking and rear bumper braking.


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