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Old 9th March 2007, 10:31   #16
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Originally Posted by rohitbagai View Post
If you mean you saving fuel then its just opposite. Engine Braking will consume more fuel than if we were accelerating at same speed in same gear. (Especially if its petrol car))
If your rpm is above the idle level and your foot is off the throttle (TPS reads closed) no fuel is injected into the cylinders. So you do save fuel.
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Old 9th March 2007, 11:13   #17
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There is no doubt that frequent short journeys have a detrimental effect on the car. This constitutes "severe service". So what is the solution to this problem? 1. Change the oil more often. 2. Take the long way back home (and burn more fuel?). Of course the second point is stupid but if you are going to get stressed out (theres no need to) about the car doing short journeys, then it may be a solution to put your mind at ease.

If you have no choice but to do short journeys then don't bother. Todays cars are quite reliable and unless you plan to keep the car for a hundred years theres no way the car is going to give up on you because it has done "short journeys". Get the car serviced and oil changed every six months and the car will serve you well.
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Old 9th March 2007, 11:16   #18
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Originally Posted by Pankaj401 View Post
If your rpm is above the idle level and your foot is off the throttle (TPS reads closed) no fuel is injected into the cylinders. So you do save fuel.
If the foot is off the throttle its the air flow thats restricted and not the fuel....and BTW if no fuel is injected into cylinders how does an engine keep alive (running)

Last edited by rohitbagai : 9th March 2007 at 11:28.
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Old 9th March 2007, 13:58   #19
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Originally Posted by adya33 View Post
I for one don't think short journeys make any difference to engine.
Really thinking about it, well maintained engines last really long.
The only problem I can think of is, lower FE since engine would be cooling when car is switched off & would again require getting warmed once started.


And based on what you would define "short journey" ?
For someone living in city a drive of 3km might take half an hour, so its definitely not a short journey for him. Where as for someone living in small town a journey of 10km might not take more than 15 min.

So how would you define short journeys?



Actually on downhill roads you should use maximum practically possible gear without pressing accelerator, as at higher gear the engine will sip less fuel resulting in better FE.
Here are some links that throw light on engine wear and FE affects of short trips obtained by googling.

Mechanisms of Engine Wear and Engine Oil Degradation in Vehicles Using M85 Or Gasoline
STP Oil FAQs
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Old 9th March 2007, 14:42   #20
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>> if no fuel is injected into cylinders how does an engine keep alive (running)

The wheels spin the engine... instead of the other way round. That's the 'funda' of engine braking. The engine now acts as a load on the car, and keeps the speed under control. It happens because the air inside the engine keeps getting compressed-decompressed at a high speed. The engine acts as an air-spring.

To the best of my knowledge, no extra fuel goes into the engine, and that's why you get the 'braking'. If there was any fuel going in, the engine would then be powering the wheels. Try this. Press the gas during engine braking, and the car comes to life in a second.

The only ill effect of prolonged engine braking is the excessive heat generated in the engine. As the air keeps getting compressed again and again, it heats up and so does the engine. But the cooling system should be able to handle that pretty well.

Last edited by amu1983 : 9th March 2007 at 14:58.
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Old 9th March 2007, 16:33   #21
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Originally Posted by rohitbagai View Post
Engine Braking will consume more fuel than if we were accelerating at same speed in same gear. (Especially if its petrol car))
Interesting... can you please explain why does it happen so?
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Old 9th March 2007, 16:41   #22
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Quote:
Originally Posted by amu1983 View Post
>> if no fuel is injected into cylinders how does an engine keep alive (running)

The wheels spin the engine... instead of the other way round. That's the 'funda' of engine braking. The engine now acts as a load on the car, and keeps the speed under control. It happens because the air inside the engine keeps getting compressed-decompressed at a high speed. The engine acts as an air-spring.

To the best of my knowledge, no extra fuel goes into the engine, and that's why you get the 'braking'. If there was any fuel going in, the engine would then be powering the wheels. Try this. Press the gas during engine braking, and the car comes to life in a second.

The only ill effect of prolonged engine braking is the excessive heat generated in the engine. As the air keeps getting compressed again and again, it heats up and so does the engine. But the cooling system should be able to handle that pretty well.
I am unsure about no fuel going in to the engine. Logically engine braking happens when the car is in lower gears (or) present gear with no acceleration. So the fuel supply is still there but less than that is needed to maintain speed. Its actually the tranny/ wheel combination loading the engine with less power, may be a little more than fuel consumption at idle RPMs.

I do not understand why the fuel would be cut off, fuel continues to be supplied even when the engine is idling (no pressure on as pedal).
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Old 9th March 2007, 16:56   #23
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Opps! I did not realize that this thread had 2nd page too, before posting previously. The question to Rohit is still valid, though.
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Old 9th March 2007, 17:04   #24
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Originally Posted by rohitbagai View Post
If the foot is off the throttle its the air flow thats restricted and not the fuel....and BTW if no fuel is injected into cylinders how does an engine keep alive (running)
Moving the pistons without burning fuel requires energy. And that energy is sucked up from the momentum of the car. This reduces the speed and is called engine braking.

here is what honda listed for it's 96-00 Civic model.

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Old 9th March 2007, 17:06   #25
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Originally Posted by amu1983 View Post
>> The only ill effect of prolonged engine braking is the excessive heat generated in the engine. As the air keeps getting compressed again and again, it heats up and so does the engine. But the cooling system should be able to handle that pretty well.
More heat is generated when your burn fuel compared to just compressing the air.
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Old 9th March 2007, 17:42   #26
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Originally Posted by navdeep View Post
I think he means the resistance provided by lower gears.
Even i think exactly this

Amul, please clarify
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Old 9th March 2007, 17:46   #27
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Originally Posted by Pankaj401 View Post
Moving the pistons without burning fuel requires energy. And that energy is sucked up from the momentum of the car. This reduces the speed and is called engine braking.

here is what honda listed for it's 96-00 Civic model.

This seems like an intelligent control mechanism, do other car makers follow this ?
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Old 9th March 2007, 17:50   #28
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Originally Posted by typeOnegative View Post
What was that? Did not get it. Do you have gas assisted brakes or am I missing something very profound?
With the ignition off, does the car move even if down a slope?
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Old 9th March 2007, 17:53   #29
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With the ignition off, does the car move even if down a slope?
Why not if it's in neutral?
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Old 9th March 2007, 17:56   #30
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Originally Posted by sbasak View Post
Why not if it's in neutral?
Sorry, I meant on 2nd gear or any other gear. Someone mentioned he goes downhill on 2nd gear without using gas.

Last edited by prabuddhadg : 9th March 2007 at 18:03.
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