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Old 16th September 2011, 12:29   #1
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Default Which has more Engine Braking? A petrol or a diesel engine?

In midst of a discussion on another thread, this came up as rather off-topic.

Which has more Engine braking everything else remaining same. A Diesel Engine or a Petrol Engine.

I used to think Diesel engines have more braking until I came across this Wikipedia entry: Engine braking - Wikipedia, the free encyclopedia

Petrol (gasoline) engines

The term engine braking usually refers to the braking effect caused by throttle position induced vacuum in petrol (gasoline) engines. While some of the braking force is due to friction in the drive train, this is negligible compared to the effect from vacuum.
When the throttle is lifted less air is allowed to pass through the intake manifold, and the engine works against this vacuum. It is the deceleration of the engine against this vacuum which provides the braking effect.
Diesel engines

Diesel engines do not maintain a throttle vacuum as they do not have an intake throttle. The fuel itself is the throttle, and thus diesel engines are not subject to the same engine braking effects as gasoline engines are.
But the thing is, I feel the statement is fundamentally flawed.

My logic was :

say a 1.4 L diesel produces X bhp at 3000 rpm and a 1.4 L petrol produces as X bhp at 5000 rpm so at the flywheel, which engines is producing more power per rpm? So if you take your leg off the gas pedal, which engine consumes more power from tranny per RPM? So which engine has more braking per RPM?

Even if you want to go stroke by stroke, it is the compression stroke that causes engine braking rather than intake stroke as mentioned there, irrespective of it is Petrol or Diesel.

Also, while an average Petrol's heads compression ratio is at 10:1, a similar Diesel would require 20:1 explaining the peak power output at almost 1/2 RPM values.

So guys please let me know- Am I thinking right? The house is now open for debate!
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Old 16th September 2011, 13:20   #2
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Default Re: Which has more Engine Braking? A petrol or a diesel engine?

By all things being equal - is it the Cubic Capacity or the Power? Normally Diesel engines have less power per CC compared to Petrol engines, hence the tendency to have larger Diesels to replace a Petrol.

I am assuming that you mean same CC. Then due to higher compression the Diesel Engine will have higher engine braking. The other factors are

1. Final Gear Ratios. These may be less for Diesels, which will reduce breaking
2. Larger Capacity of Diesel Engine for similar power increases breaking
3. The ECU has a major influence on braking, as in some cases the ECU may cut off the fuel when the foot is off the accelerator pedal, in other cases ECU may allow enough fuel for the engine to idle, or the ECU may reduce the fuel at high speed, while allowing enough fuel to crawl at lower speeds. All this affects the braking effect.
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Old 16th September 2011, 13:25   #3
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Default Re: Which has more Engine Braking? A petrol or a diesel engine?

This topic has been discussed here in detail-
http://www.team-bhp.com/forum/4x4-te...e-braking.html (Unable to achieve engine braking)

Also check out this post for engine braking vs gear braking-
http://www.team-bhp.com/forum/4x4-te...ml#post1968838 (Unable to achieve engine braking)
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Old 16th September 2011, 14:47   #4
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Default Re: Which has more Engine Braking? A petrol or a diesel engine?

Note from the Team-BHP Support Team: Topic already exists. Please search before creating new thread.
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