Understanding Compression Ratio - xBhp.com : The Global Indian Biking Community

Honda_CBF · 01-30-2012, 10:46 PM

Note to moderators:
I tried searching for related topic but wasn't able to find satisfactory results, so started this thread. Plz move or attach if needed.
The topic:
I know what compression ratio is, i.e. In a piston engine it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke , and the volume of thecombustion chamber when the piston is at the top of its stroke.
.
But I want know its effects when altered (increased or decreased) with the respect to torque/horsepower/fuel efficiency??
I also tried googling for it but they are contradictory and related to the huge engined cars.
So plz experts help me understand it all..

sunilg · 01-31-2012, 03:40 PM

Discussion approved.

ROCKRZ · 01-31-2012, 03:50 PM

the explaination from wikipedia:-

Quote:

The 'compression ratio' of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity. It is a fundamental specification for many common combustion engines.
In a piston engine it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.[1]
Picture a cylinder and its combustion chamber with the piston at the bottom of its stroke containing 1000 cc of air (900 cc in the cylinder plus 100 cc in the combustion chamber). When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be proportionally described as 1000:100, or with fractional reduction, a 10:1 compression ratio.
A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency.[citation needed] High ratios place the available oxygen and fuel molecules into a reduced space along with the adiabatic heat of compression–causing better mixing and evaporation of the fuel droplets.[citation needed] Thus they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree.[citation needed]
Higher compression ratios will however make gasoline engines subject to engine knocking if lower octane-rated fuel is used, also known as detonation. This can reduce efficiency or damage the engine if knock sensors are not present to retard the timing. However, knock sensors have been a requirement of the OBD-II specification used in 1996 model year vehicles and newer.
Diesel engines on the other hand operate on the principle of compression ignition, so that a fuel which resists autoignition will cause late ignition which will also lead to engine knock.

in short, if you compress 10 cc (cubic centimeter) of air to 1 cc, you have compressed the air by ratio of 10:1 this is exactly what the engine compression ratio means..

in ideal condition, the relation between Pressure (P) and volume is P*V=nRT

tusharsmoily · 01-31-2012, 08:55 PM

The textbook defintion of Compression Ratio-

It is the ratio of the volume of cylinder when the pistion is at the end of suction stroke (BDC) to the volume of cylinder when the piston is at the end of compression stroke (TDC). Higher the CR, more is the fuel efficiency and power. But it should kept upto a critical value, beyond which, will lead to unpleasant explosion called 'knocking'. It damages the engine.

Honda_CBF · 02-01-2012, 01:50 PM

Quote:

Originally Posted by sunilg

Discussion approved.

Thanks for approval sir

@ROCKRZ and tusharmoily,
Thanks for the explanations and definations guys.
But my concern was (and still is)

Quote:

Originally Posted by Honda_CBF

But I want know its effects when altered (increased or decreased) with the respect to torque/horsepower/fuel efficiency??

for instance I'm asking what will happen to ZMR if its compression is increased to 10:1 or say, what will happen to R15 or P220 if their compression is lowered to 7:1

I know doing so can kill these bikes but I want to know how it changes its power delivery & mileage and why?
I also searched about playing with compression ratio to increase mileage and power but again results were unsatisfactory.. few sites/articles said increasing compression will help gain power & mileage both but other said it will kill mileage and provide only power gain.

aargee · 02-01-2012, 02:18 PM

Quote:

Originally Posted by Honda_CBF

for instance I'm asking what will happen to ZMR if its compression is increased to 10:1

Think...when CR increases so does pressure, so does temperature, so does power.

Quote:

Originally Posted by Honda_CBF

what will happen to R15 or P220 if their compression is lowered to 7:1

The reverse of what I said above

Quote:

Originally Posted by Honda_CBF

how it changes its power delivery & mileage and why?

How? When pressure drops, power drops, so for the same volume of fuel, the power is less & hence FE drops

Quote:

Originally Posted by Honda_CBF

other said it will kill mileage and provide only power gain.

True to an extent, unless they're not an overkill. Too much CR causes pre ignition (fuel burns even before the spark due to pressure & rise in temperature).

Give a thought on this...
The 848 Evo has a V-Twin (rather L-Twin) with a high CR around 13:1 & less than 1200cc, while MT01 is a 1800 motor with a CR around 8:1; consider which is faster, has more power & why's evo a LC engine while other is not. You can find most answers by comparing both of these.

Honda_CBF · 02-01-2012, 02:32 PM

^^ okay, got an idea of power and pressure. Any thing regarding increasing FE via altering CR??
Any sure way for landing onto win-win position?? Any calculation?

aargee · 02-01-2012, 02:51 PM

Quote:

Originally Posted by Honda_CBF

Any thing regarding increasing FE via altering CR??

Would you need more on...

Quote:

How? When pressure drops, power drops, so for the same volume of fuel, the power is less & hence FE drops

Quote:

Originally Posted by Honda_CBF

Any sure way for landing onto win-win position?? Any calculation?

Win-win position of what? And what calculation are we talking about here?

Honda_CBF · 02-01-2012, 03:12 PM

Quote:

Originally Posted by aargee

Think...when CR increases so does pressure, so does temperature, so does power.

The reverse of what I said above

When pressure drops, power drops, so for the same volume of fuel, the power is less & hence FE drops

that's what i thought/understood,
but some articles say decrease in CR = gain FE & loose power.!! This 'increased FE' part is what I'm not able to get.

And I was talking about the calculations to choose best CR for increasing power & mileage & durability (also considering 90 octain fuel; taking care of detonation.)

Drifty · 02-01-2012, 06:26 PM

Quote:

Originally Posted by Honda_CBF

that's what i thought/understood,
but some articles say decrease in CR = gain FE & loose power.!! This 'increased FE' part is what I'm not able to get.

And I was talking about the calculations to choose best CR for increasing power & mileage & durability (also considering 90 octain fuel; taking care of detonation.)

You have a good example of the Joel tuned bikes which by altering compression ratio also gives good fuel efficiency as reported by many users.
I think this query will be best answered by Joel!

01-30-2012, 10:46 PM	#1 (permalink)
Honda_CBF Still learning the art of bike'in.. Join Date: Jan 2011 Location: Jamshedpur, Jharkhand Posts: 692	Understanding Compression Ratio Note to moderators: I tried searching for related topic but wasn't able to find satisfactory results, so started this thread. Plz move or attach if needed. The topic: I know what compression ratio is, i.e. In a piston engine it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke , and the volume of thecombustion chamber when the piston is at the top of its stroke. . But I want know its effects when altered (increased or decreased) with the respect to torque/horsepower/fuel efficiency?? I also tried googling for it but they are contradictory and related to the huge engined cars. So plz experts help me understand it all.. __________________ http://www.facebook.com/ateesh.kumar Last edited by Honda_CBF; 02-01-2012 at 01:34 PM.

01-31-2012, 03:40 PM	#2 (permalink)
sunilg Super Moderator Join Date: Apr 2004 Location: Ghaziabad, UP, India. Posts: 5,075	Discussion approved. __________________ (Been There Done That) x 3.25 Join xBhp On

02-01-2012, 02:32 PM	#7 (permalink)
Honda_CBF Still learning the art of bike'in.. Join Date: Jan 2011 Location: Jamshedpur, Jharkhand Posts: 692	^^ okay, got an idea of power and pressure. Any thing regarding increasing FE via altering CR?? Any sure way for landing onto win-win position?? Any calculation? __________________ http://www.facebook.com/ateesh.kumar

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01-31-2012, 08:55 PM	#4 (permalink)
tusharsmoily Senior Member Join Date: Dec 2010 Posts: 108	The textbook defintion of Compression Ratio- It is the ratio of the volume of cylinder when the pistion is at the end of suction stroke (BDC) to the volume of cylinder when the piston is at the end of compression stroke (TDC). Higher the CR, more is the fuel efficiency and power. But it should kept upto a critical value, beyond which, will lead to unpleasant explosion called 'knocking'. It damages the engine.

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