True or False?: You have two engines that are built identically, except for the size of the combustion chambers in each respective engine's heads. Engine A has 10:1 compression. Engine B has 11:1 compression. You feed one gallon of 93 octane gasoline to each engine, and run them both at idle speed. The 10:1 engine runs out of gas faster...

 

My guess is, the 10:1 engine will be less efficient at idle. The lack of 'assist'
from the lower compression ratio will require more effort from the ignition.

Since VE and BMEP values are going to be relatively lower compared to the 11:1 motor, the engine is going to work harder to move the pistons per volume
of intake charge taken in.

 

I would say the lower comp engine would be the engine to run longer it has less compression to overcome and thus works less to run at idle
ignition would have to work harder on the high compression engine to overcome the the higher compression to jump the spark
the higher compression engine will produce more heat and more friction
than the lower comperession engine and depending on the iduction system if carb the higher compression engine would require more fuel to run as cool as the lower compression engine

 

Interesting angle. Wouldn't the lower cylinder pressure require more effort to
ignite and combust the mixture?



I guess we're talking about very subtle changes, however at low RPM isn't
the effective compression so low that it hinders complete combustion?

IOW, fuel needs to be a certain temperature to combust evenly.

 

True,

The single most important mechanical factor in the efficency of a engine is the compression ratio.

The difference of the temperature / presssure of the chamber at the top and bottom of the stroke is the energy that is used to move the piston.

This temperature / pressure change is larger when the CR is higher. Since the change in volume is greater. Greater volume change means greater temperature / pressure change.

This will be true at all engine rpm, if all other factors are the same.

Diesels run 23:1 CR and no throttle plate. Part throttle fuel efficency is what they are all about.

Z28

 

moreso on the higher compression the coil has to work harder to complet the spark and the added force needed to build the higher comperression
will cause more friction thus needing more energy to complete all 4 strokes

 

by the way i just cheked out your car why dont you come to cayuga on the 10th its A amc day but you can run anything im taking the mighty Z to kill a few now that ive redone the engine i should be able to run low 13s

 

Theoretically, the higher the compression the higher the efficiency. Engine efficiency formulas are all based off of compression.

Ben

 

cumbution chamber shape determins efficiency not compression
if both engine are identical except compression the pistons are
changed to bump compression so the higher compression engine is no
more efficient than the lower compression motor with the same cumbusion chamber

 

They are both factors. Furthermore, the REAL question was; 'Will raising the compression ratio increase or reduce Brake Specific Fuel Consumption?'.

 

Sorry, but that is an incorrect statement.

There will be power gains if the higher compression doesn't cause any new problems.

Z28

 

Think about an idling engine:

The only brake load on it is the acessory drive and either turning over the transmission gears in neutral (manual) or running the transmission pump and spinning some gears (automatic). Basically it is just producing enough power to overcome its own friction and pumping losses.

With the throttle nearly closed, very little air is getting into the engine, so it probably has a VE close to single digits. That limits the effective compression to something like a model T Ford's static CR of 4:1...maybe less. No way does it need 93 octane gas to idle, or 87 or even lower.

The higher the effective compression, the more power the engine makes from each bit of fuel, so the 11:1 engine might do better if it's EFI, with O2 feedback and the amount of fuel needed is being adjusted by the PCM. To hold the given idle speed it will need to inject less fuel to the 11:1 engine.

IMO, more effective compression, and in this hypothetical case, more SCR = more "effective" compression at idle, it will take less fuel to hold a given idle speed with the 11:1 engine.

I'm not sure why you would want to idle this engine thru a gallon of fuel with prices approacing $4/gal for 93. If the engine is close to stock, it's going to take quite a while to idle away a gallon. Maybe you like to park and watch the submarine races.

Perhaps looking at part throttle cruising, which is making the engine make a little "brake" power, say 30 hp or so would be another way to look at whether the 11:1 or 10:1 engine would get farther down the road on a gallon.

Back in pre-emission, carb only days, higher compression ratio engines, otherwise identical to lower compression versions gave more mpg in controlled level road fuel economy tests. Was there-did that.

spinout, no offense, but you have a few mistaken notions.

My $.02.

 

i agree, my .0002.
this is why one can drive their 12 to 14:1 car around town on pump gas or to the "ahem" track...just dont get WOT or above say 5K.


im gonna try and connect this to why higher CR motors have crisper throttle response in similar applications, so bear with me.so if you are using less fuel with the higher CR motor, and getting a more complete and efficient burn, when you add fuel there is less of a transitional rich/"bogging" period as the motor can more efficiently ingest fuel and consume it.
no? think so? maybe?
maybe someone can better explain what im trying to say.....ive been on a plane since 630, and am not firing on all 8.

 

You might find it interesting to read about SAAB's Variable Compression engine

 

Interesting little note, when designing air compressors, 9:1 is considered the most energy efficient. The higher the compression, the more energy required to compress. Might also be why so many modern engines are designed in this area.