Accelerator query

[tigermad]

I have just about got used to my 05 reg Octy II 2.0 tdi. One query though.

I notice compared to other cars I have driven that when you take your foot off the accelator the speed decreases quite rapidly. For instance sometimes when I am doing 50mph in 4th and I change gear to 5th the speed will drop a few miles per hour and end up slower than 50mph. Is this normal?

[Taxi 219]

Yes it seems perfectly normal to me.Of course by changing gear you would lift off first so obviouslyyou would lose a bit of momentum.

[androdev]

my experience is quite the opposite. when i let go off the accelerator, there is very negligible speed drop compared to the other (petrol) cars i have driven. this i was told is normal as diesels have very minimal engine breaking.

in fact, at very low speeds the car gains momentum when i let go off the accelerator and depress the clutch fully.

[bahnstormer vrs]

Originally Posted by tigermad

I have just about got used to my 05 reg Octy II 2.0 tdi. One query though.

I notice compared to other cars I have driven that when you take your foot off the accelerator the speed decreases quite rapidly. For instance sometimes when I am doing 50mph in 4th and I change gear to 5th the speed will drop a few miles per hour and end up slower than 50mph. Is this normal?

Don't think so; but some loss of speed is inevitable due to the lack of drive pushing the car forward.

Have you got the handbrake on? Going up hill? headwind? Changing gear leisurely?

[bahnstormer vrs]

Originally Posted by androdev

my experience is quite the opposite. when i let go off the accelerator, there is very negligible speed drop compared to the other (petrol) cars i have driven. this i was told is normal as diesels have very minimal engine breaking.

in fact, at very low speeds the car gains momentum when i let go off the accelerator and depress the clutch fully.

Sorry; But you have surely got these points back to front!

Diesel engines have a higher compression ratio compared to petrol, typically 20:1 - 22:1 vs 9:1 - 11:1 and thus a diesel engine will provide MORE engine braking.

Gaining momentum without the engine providing forward power is against the laws of physics unless there is another influence i.e. gravity (going downhill) providing a greater power than the engine.

[mannyo]

To the OP, you proberbly just need to adapt your driving style. Driving a diesel is very different to a petrol, I found it took me a while to get the best out the engine/gearbox.

[androdev]

Originally Posted by bahnstormer vrs

Sorry; But you have surely got these points back to front!

Diesel engines have a higher compression ratio compared to petrol, typically 20:1 - 22:1 vs 9:1 - 11:1 and thus a diesel engine will provide MORE engine braking.

This is what I expected but then I did some googling to find out why I don't notice engine braking in my diesel. Here is a quote from wiki Jake brake - Wikipedia, the free encyclopedia

When the accelerator is released on a truck, its forward momentum continues to turn the crankshaft and compress air inside the engine's cylinders. When the crankshaft passes the top-dead-center position, the compressed air in the cylinder acts as a spring and pushes the piston back down the cylinder, returning the energy to the crankshaft and pushing the truck forward. Little of the energy absorbed by compressing the air is lost, so the engine does not effectively aid in slowing the truck. Of equal importance, even with zero accelerator input, there will be some trace introduction of diesel fuel (make and model dependent) which will still undergo combustion. Any power created from this will hinder efforts to decelerate.

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.

Gaining momentum without the engine providing forward power is against the laws of physics unless there is another influence i.e. gravity (going downhill) providing a greater power than the engine.

Could be downhill but not much. It used be to pretty bad while driving in bumper-to-bumper traffic as I had some close shaves. Now I use brakes :-) All I am trying to say is there is no engine braking happening and I had to adapt my driving style accordingly.

[Jules59]

Well all I can say is you just try and push a diesel car in gear and compare it to petrol engine - you wont get the diesel very far.
The comment from wiki:

...When the crankshaft passes the top-dead-center position, the compressed air in the cylinder acts as a spring and pushes the piston back down the cylinder...

is true of petrol engines as well. Its just that in a diesel to reach TDC causes a much higher cylinder pressure to be reached which is the "braking effect".

If lorrys didnt have engine braking why are they supposed to change down to lower gears when descending steep hills - the engine braking prevents brake fade from the drums overheating and expanding.

[Lummox]

Originally Posted by Jules59

If lorrys didnt have engine braking why are they supposed to change down to lower gears when descending steep hills - the engine braking prevents brake fade from the drums overheating and expanding.

Thats why they have an exhaust brake that generates engine braking.

[Jules59]

I didnt know that. how does an exhaust brake work ?
My landrovers (Ive had a few) all had pretty impressive engine braking without an exhaust brake that I know off - but then again they only weighed a couple of tonnes. they autobox ones would lock up the torque convertor to prevent freewheeling like a normal car automatic - clever i thought .

[Organist]

Originally Posted by Lummox

Thats why they have an exhaust brake that generates engine braking.

........or a Telmar retarder acting on the propshaft.