Driving Economically.

mikey72

alastairq wrote: »

........... have greater 'braking' effect than the equivalent petrol engines....it is the compression that creates the braking effect.




..............

</p>That's about as wrong as it gets. The diesel engine compresses the air in a cylinder, then uses the stored enegy in the compressed air to move the piston back down, and compress the next cylinder with it. So very little braking effect is achieved. A petrol by comparison, has to contend with the vacuum created by the closed throttle body, this is in fact what creates the braking, as the piston tries to overcome this, and loses energy. I think your impression of lack of engine braking is because there is a lack of engine braking. Maybe you felt what you wanted to believe comparing both, and didn't really realise what was happening with the two different engines and vehicles?

alastairq

maybe so...however, the practical aspects I have suggested still stand......a driver needs to find out how to best utilise the fact that..on a modern diesel engine [common rail especially] as well as a petrol engine..on over-run, the fuel is returned to the tank..ie unused.

Lots of hints and tips abound, but the real savings in fuel..when comparing a modern engine, especially electronically controlled, as against older technology [with mechanical control]...lie with the ability to not use fuel under over-run conditions.

[I include Bosche K-jetronic as a modern technology, however]

In fact, the apparent lack of engine braking available from a diesel engine [not my experience, I hasten to add] is of benefit to a driver, since it would prolongue the rate of speed loss even further........the further a car can travel off the gas, the more fuel saved?

Certainly as against the opposite option, of driving under power up to a point where the brakes need to be used to reduce speed to the new intended level.

Lum

I think it's kind of pointless to argue whether petrol or diesel provides more or less engine braking as it's going to vary more by the type of engine, gearbox and the car it's hooked up to than it is by engine fuel type.

FWIW, My 1993 automatic turbo petrol car has a lot more engine braking than my 2010 manual turbo diesel car.

mikey72

Lum wrote: »

I think it's kind of pointless to argue whether petrol or diesel provides more or less engine braking as it's going to vary more by the type of engine, gearbox and the car it's hooked up to than it is by engine fuel type.

FWIW, My 1993 automatic turbo petrol car has a lot more engine braking than my 2010 manual turbo diesel car.

We'll have to agree to differ then, as petrol has it, and diesel doesn't. That's more fundamental than anything else, and greatly affects approaching junctions on or off throttle, without needing to use brakes etc, which is were we came in, and where the tips on driving started.

Lum

Sorry, I should have been more clear. I mean the overall braking on overrun is more affected by gearbox, car design etc.

If a given diesel car has the aerodynamics of a brick and uses a torque converter auto it's probably going to decelerate quicker than a petrol car with a manual gearbox and good aerodynamics.

mikey72

A diesel on a fluid couple torque converter would have to be tied to a barn door to match a manual petrol even shaped like thrust 2. If you mean an automatic petrol, that should still be better than a manual diesel, all other things being equal. If you want to look at engine braking on a diesel, look at jake brakes for a way to do it.

grizzly1911

mikey72 wrote: »

A diesel on a fluid couple torque converter would have to be tied to a barn door to match a manual petrol even shaped like thrust 2. If you mean an automatic petrol, that should still be better than a manual diesel, all other things being equal. If you want to look at engine braking on a diesel, look at jake brakes for a way to do it.

Many years ago (child)I travelled in in old coach and sat behind the driver.

I watched him activating a lever on the steering column, which was part of the braking, retarding process especially on long downhills - found it fascinating - sad I know.

I thought he was closing down the exhaust to create back pressure but perhaps that was a Jacobs Brake (wiki interesting):think:

alastairq

I watched him activating a lever on the steering column

that would be a 'retarder'....of which there are many types....all of which have the objective of slowing the vehicle but not using the 'wheel' brakes.

There have been many types of retarders...'exhaust brakes' being the most commonly heard one's these days.


Your example may well have been something like a 'Telma' retarder...which worked a bit like an electric motor....on the propshaft......[prop surrounded by coils of wire, with maybe magnets on prop itself.....electric current applied to coils, try to 'turn' the magnets [on prop] the opposite way.]....slowing [retarding] the prop, hence the vehicle.....

there were/are many others...including a device on auto and semi-auto epicyclic gearboxes, which sort-of gradually engages reverse gear to slow the plot down...



The issue of whether diesel engines display engine braking or not really isn't the point.


Explain why every diesel car, 4x4, truck or bus I've driven, slows down whenever I came ''off the gas?''


Why, when confronted with a steep down hill, in a lorry bus or car, I would slow, engage low gear, and let the engine 'lower' me down the hill safely?


Or why, when driving 'off-road', and confronted with a steep down hill or hole, I would engage a low gear [if not the lowest gear] to lower the vehicle down, on the engine..... without resort to the footbrake [as that will create problems in all likelihood?}


The fact is, regardless of what fuels the engine, off the 'gas ' pedal means the vehicle starts to slow down.

The lower the gear when off the gas, the more pronounced the rate of slowing.


As I said, my objective is to see how far my car can go, off the gas..[ie not using fuel]...at every opportunity.......and which doesn't make me a 'slow'driver ....certainly by the standards displayed on these boards?

A simple concept of driving, taking advantage of modern technology.

And vastly different to the economy-driving techniques I would have used in the days when my vehicles had carburettors.

almillar

mikey72 - airflow? What's that got to do with it?Diesel cars absolutely DO have engine braking. The engine, no matter which one, with no throttle applied, will slow down, dragging the gearbox, wheels and car down with it.
I've driven a few petrols and diesels, and TBH being grossly general, there's not much difference in the engine braking between the 2, rev for rev.

mikey72

almillar wrote: »

mikey72 - airflow? What's that got to do with it?Diesel cars absolutely DO have engine braking. The engine, no matter which one, with no throttle applied, will slow down, dragging the gearbox, wheels and car down with it.
I've driven a few petrols and diesels, and TBH being grossly general, there's not much difference in the engine braking between the 2, rev for rev.

Airflow, well lack of it, is the main cause of engine braking, diesels don't lack airflow, petrols do when you lift off.