What is right mechanism to reduce speed?

prabuddhadg · 14th July 2008, 16:20

Quote:

Originally Posted by Rehaan

....Think about this - in a 4cyl engine one piston is on the compression stroke which is definitely causing resistance since it is squishing the volume of air to about 1/10th its volume (eg. 10:1 compression ratio), BUT at the same time another cylinder is on the power stroke (but there is no detonation because the drivers foot is not on the gas during engine braking), so the air in that cylinder is expanding to about 10times its squished volume - which is providing some force pushing down on the piston - and hence cancelling out the compression stroke resistance!

End result? In theory the compression/expansion cancel each other out, so you are left with only friction!

Agree with all the rest, but not with the sentence in bold. Plugs will spark whether the driver's foot is on the gas or not. As long as electricity is flowing, petrol engines will have detonation. And as long as there is compression, diesel engines will detonate.

DKG · 14th July 2008, 17:06

As students of physics we've all heard about kinetic and potential energy. A car travelling at speed has a certain quantum of energy manifesting as speed. Since in nature energy remains constant you need to transform energy if you wish to reduce any one form.

What that means is if you wish to reduce speed you either use brakes which transform speed into heat or use the gearbox and engine in sync to further transform the speed energy into heat. Ideally if you use both in tandem you transform the energy (speed) faster into heat, thereby reducing the speed.

Therefore to effectively reduce speed you must use any means possible to dissipate/transform one form of energy (speed) into another (heat). In the case of a car needing to be slowed down using brakes and the engine is the most effective way to slow down a car.

Today in hybrid cars instead of wasting the potential energy of speed into heat during braking, manufacturers are transforming it into electricity to recharge batteries. So in a hybrid the moment you lift your foot off the accelerator the electric motor uses the kinetic energy of the wheels to generate electricity

One last point, someone mentioned about combustion during engine braking. This is not the case even in old cars as with your foot off the accelerator the engine is starved of fuel/air mixture so it retards combustion. Engine braking efficiently retards movement. Infact vaccum plays a major role in deceleration, as the engine is starved of air (throttle bodies closed due to your foot off the accelerator)

prabuddhadg · 14th July 2008, 18:20

I want to understand one thing. When the foot is off the accelerator, the engine still receives enough fuel to be able to idle, right? So, there must be idle level of combustion. I am only speaking from common sense, and do not know much about tech stuff. I thought, with carb engines, while the accelerator is being pressed, there is extra supply of fuel into the engine. At all other times, the steady stream of fuel keeps the combustion going. With mpfi engines, I guess the on board computer may be able to cut off fuel supply at such times- I don't know. Can the experts help out?

Shan2nu · 14th July 2008, 22:50

Quote:

As per my understanding this is not really correct Shan.

The MAIN reason for engine braking is FRICTION (not the compression of air).

Think about this - in a 4cyl engine one piston is on the compression stroke which is definitely causing resistance since it is squishing the volume of air to about 1/10th its volume (eg. 10:1 compression ratio), BUT at the same time another cylinder is on the power stroke (but there is no detonation because the drivers foot is not on the gas during engine braking), so the air in that cylinder is expanding to about 10times its squished volume - which is providing some force pushing down on the piston - and hence cancelling out the compression stroke resistance!

End result? In theory the compression/expansion cancel each other out, so you are left with only friction!

Hmm, you got a point there but what i don't understand is, does the engine actually produce that much friction? Just imagine the amount of power being wasted just to overcome that frictional loss when acc.

Quote:

Once again, even if those cylinders were working in vaccuum, (it would be the other way around) the power stroke would be causing resistance and the compression stroke would be aided by the original vaccum.

However, think about this - what if the exhaust valve opened after the compression stroke and then closed leaving a lower pressuse in the cylinder just in time for the (unfueled) power stroke - that would be a win-win for retardation! There would be resistance on the pistons way up (squishing air) AND on the way down (sucking on low pressure), and that is the principle on which jake-brakes (also known as compression-release brakes) that are found of heavy vehichles work!

But thats how VCM works, it shuts off the valves on half the number of cylinders. How they overcome the vaccuum hasn't been explained anywhere.

Variable Cylinder Management - Wikipedia, the free encyclopedia

Shan2nu

proton · 17th July 2008, 18:57

My 2 paise worth!

Just think of the engine as a compressor attached to the drive train. Any factor that reduces the efficiency of the compressor will cause the braking to increase.

Now to muddy the water a little more:

A 4 stroke has got two strokes that are effectively not pushing air through the cylinders. These would account for the increased braking available with 4 stroke engines.

Today, with the high speeds involved, the main factor affecting braking is adhesion. Without adhesion, you have no braking (and to a certain extent, no steering). Loss of adhesion is due to transfer of weight to front wheels and subsequent exceeding of the limits of the rubber to provide braking.

Solution: ensure limits of rubber are not exceeded.

Technology: ABS, which senses wheel lockup and "pumps" braking mechanism far better than human sensing and responding. Not to be confused with traction control.

Bottom line: Use engine braking to avoid fading (not so much of a factor with all wheel disc braking systems). This way you save your brakes for the time you really need maximum stopping power.

Use pumping when faced with lockup during emergency panic stops.

Invest in ABS. This is probably one of the best advances in automotive technology to have come out of research labs.

RX135 · 18th July 2008, 13:43

Quote:

Originally Posted by Rehaan

As per my understanding this is not really correct Shan.

The MAIN reason for engine braking is FRICTION (not the compression of air).

Think about this - in a 4cyl engine one piston is on the compression stroke which is definitely causing resistance since it is squishing the volume of air to about 1/10th its volume (eg. 10:1 compression ratio), BUT at the same time another cylinder is on the power stroke (but there is no detonation because the drivers foot is not on the gas during engine braking), so the air in that cylinder is expanding to about 10times its squished volume - which is providing some force pushing down on the piston - and hence cancelling out the compression stroke resistance!

End result? In theory the compression/expansion cancel each other out, so you are left with only friction!

Rehaan,

I am not entirely convinced with this argument about Friction. You need to consider all 4 strokes.

1st stroke is suction, where piston needs to create vaccum for pulling in external air. Work is done to achieve this. External force required.
2nd stroke is compression, where piston needs to compress air. Work is done to achieve this. Needs external force.
3rd stroke is expansion, which happens due to compressed air, which as you say, kind of compensates for compression barring frictional losses.
4th stroke is exhause, where piston is pushing the air out of engine. Again requires external force.

Even assuming that, there is 0 frictional loss, work is required done to complete 1 Otto cycle. This is provided by kinetic energy of moving vehicle.

Hope, this clears it up.

Rehaan · 22nd July 2008, 09:10

Quote:

Originally Posted by prabuddhadg

Agree with all the rest, but not with the sentence in bold. Plugs will spark whether the driver's foot is on the gas or not. As long as electricity is flowing, petrol engines will have detonation.

Well, it depends on whether you are considering "detonation" to mean spark or combustion. There may or may not be spark(im not sure), but I meant combustion - which will not be happening since there will be no fuel going in.
The same as when in a wasted spark system there is no detonation on the wasted spark (since theres nothing to burn when it fires on the exhaust stroke).

Reason below -

Quote:

Originally Posted by prabuddhadg

...When the foot is off the accelerator, the engine still receives enough fuel to be able to idle, right? ...

No it doesnt.
Once again please see my first para in this post : http://www.team-bhp.com/forum/techni...tml#post226145

Quote:

Originally Posted by Shan2nu

Hmm, you got a point there but what i don't understand is, does the engine actually produce that much friction?

This is only my understanding of it as i mentioned earlier. I'm bringing it up for continued discussion and more than open to bettering my understanding of it - but what i explained is what currently makes the most sense to me. However...

Quote:

Originally Posted by Shan2nu

Just imagine the amount of power being wasted just to overcome that frictional loss when acc.

...but then again thats one of the reasons why engines are not so efficient. Even your drive train (consisting of bearings and gears) loses ~15-20% due to friction as you know. So imagine what must be happening in the engine where there are bearings and gears as well as parts like piston rings "scraping" against sleeves etc!
I know the thermodynamic efficiency of an engine is around 37% -- not sure what frictional losses are like.

Quote:

Originally Posted by RX135

Rehaan,

I am not entirely convinced with this argument about Friction. You need to consider all 4 strokes.

1st stroke is suction, where piston needs to create vaccum for pulling in external air. Work is done to achieve this. External force required.
2nd stroke is compression, where piston needs to compress air. Work is done to achieve this. Needs external force.
3rd stroke is expansion, which happens due to compressed air, which as you say, kind of compensates for compression barring frictional losses.
4th stroke is exhause, where piston is pushing the air out of engine. Again requires external force.

Even assuming that, there is 0 frictional loss, work is required done to complete 1 Otto cycle. This is provided by kinetic energy of moving vehicle.

Hope, this clears it up.

Hi RX,

Surely there are other forces adding to the engine braking other than friction (for example, even the effort to switch directions due to the inertia of the reciprocating pistons plays a part) - but i was saying that friction is the main reason & plays a larger part than compression. After thinking about it some more (with reference to your post above) i guess the breathing does play a larger part than i assumed, however :

In stroke 1(intake) and stroke 2(compression) i still think they mostly cancel each other out. The throttle would be closed.
Without friction, there really wouldnt be much retardation in these two strokes IMO.

In stroke 3(power) there is a retardation because the piston is moving downwards with a low pressure in the chamber.
Then in stroke 4 (exhaust) the exhaust valves open while the piston is still on its way up from BDC, and the chamber is still a low pressure. Who knows what happens with the flow in actuality - but i guess we could imagine that this causes "suction back into the cylinder" ? -- im just guessing in the dark here.
So i guess that does cause additional retardation/engine braking as you have said - but i would still say the major reason is friction.

Would love to discuss this further and get more viewpoints.

cya
R

greenhorn · 22nd July 2008, 09:20

Is engine braking less effective for diesels than petrols ? I remember reading something like that on an engine brake for diesels thread.

prabuddhadg · 22nd July 2008, 09:55

Quote:

Originally Posted by Rehaan

No it doesnt.
Once again please see my first para in this post : http://www.team-bhp.com/forum/techni...tml#post226145
R

Thanks Rehaan, I did not know that modern cars actually cut off all fuel supply when the car is in motion and the accelerator not pressed. That clears up a lot of the haze.
My info was a little dated I guess. Sorry for barking up the wrong tree.

Nitronium · 4th November 2008, 16:39

A doubt of my own.
Say i'm doing about 100 in 3rd, and the rpm is @ say 4500, is it alright to press the brake gently without the clutch to slow the car down, at least till about 2000 rpm( Will the engine braking assist in speed retardation?)? I tried this out, and it worked rather well. Do correct me if i'm wrong or doing my car in for good!

14th July 2008, 17:06	#77
DKG Senior - BHPian Join Date: Feb 2008 Location: Hyderabad Posts: 3,711 Thanked: 1,389 Times View My Garage	As students of physics we've all heard about kinetic and potential energy. A car travelling at speed has a certain quantum of energy manifesting as speed. Since in nature energy remains constant you need to transform energy if you wish to reduce any one form. What that means is if you wish to reduce speed you either use brakes which transform speed into heat or use the gearbox and engine in sync to further transform the speed energy into heat. Ideally if you use both in tandem you transform the energy (speed) faster into heat, thereby reducing the speed. Therefore to effectively reduce speed you must use any means possible to dissipate/transform one form of energy (speed) into another (heat). In the case of a car needing to be slowed down using brakes and the engine is the most effective way to slow down a car. Today in hybrid cars instead of wasting the potential energy of speed into heat during braking, manufacturers are transforming it into electricity to recharge batteries. So in a hybrid the moment you lift your foot off the accelerator the electric motor uses the kinetic energy of the wheels to generate electricity One last point, someone mentioned about combustion during engine braking. This is not the case even in old cars as with your foot off the accelerator the engine is starved of fuel/air mixture so it retards combustion. Engine braking efficiently retards movement. Infact vaccum plays a major role in deceleration, as the engine is starved of air (throttle bodies closed due to your foot off the accelerator) Last edited by DKG : 14th July 2008 at 17:14.
	() Thanks

14th July 2008, 18:20	#78
prabuddhadg BHPian Join Date: Jan 2006 Location: Mumbai Posts: 812 Thanked: 81 Times View My Garage	I want to understand one thing. When the foot is off the accelerator, the engine still receives enough fuel to be able to idle, right? So, there must be idle level of combustion. I am only speaking from common sense, and do not know much about tech stuff. I thought, with carb engines, while the accelerator is being pressed, there is extra supply of fuel into the engine. At all other times, the steady stream of fuel keeps the combustion going. With mpfi engines, I guess the on board computer may be able to cut off fuel supply at such times- I don't know. Can the experts help out?
	() Thanks

17th July 2008, 18:57	#80
proton BHPian Join Date: Jan 2008 Location: Bangalore Posts: 204 Thanked: 393 Times	My 2 paise worth! Just think of the engine as a compressor attached to the drive train. Any factor that reduces the efficiency of the compressor will cause the braking to increase. Now to muddy the water a little more: A 4 stroke has got two strokes that are effectively not pushing air through the cylinders. These would account for the increased braking available with 4 stroke engines. Today, with the high speeds involved, the main factor affecting braking is adhesion. Without adhesion, you have no braking (and to a certain extent, no steering). Loss of adhesion is due to transfer of weight to front wheels and subsequent exceeding of the limits of the rubber to provide braking. Solution: ensure limits of rubber are not exceeded. Technology: ABS, which senses wheel lockup and "pumps" braking mechanism far better than human sensing and responding. Not to be confused with traction control. Bottom line: Use engine braking to avoid fading (not so much of a factor with all wheel disc braking systems). This way you save your brakes for the time you really need maximum stopping power. Use pumping when faced with lockup during emergency panic stops. Invest in ABS. This is probably one of the best advances in automotive technology to have come out of research labs.
	() Thanks

22nd July 2008, 09:20	#83
greenhorn Senior - BHPian Join Date: Apr 2007 Location: KL-01 Posts: 7,745 Thanked: 4,402 Times View My Garage	Is engine braking less effective for diesels than petrols ? I remember reading something like that on an engine brake for diesels thread.
	() Thanks

4th November 2008, 16:39	#85
Nitronium BHPian Join Date: Aug 2008 Location: Bangalore Posts: 407 Thanked: 93 Times	A doubt of my own. Say i'm doing about 100 in 3rd, and the rpm is @ say 4500, is it alright to press the brake gently without the clutch to slow the car down, at least till about 2000 rpm( Will the engine braking assist in speed retardation?)? I tried this out, and it worked rather well. Do correct me if i'm wrong or doing my car in for good!
	() Thanks