exhausts effect on fuel consumption

s-cogan

howr ye keeping lads,
Im driving a 1,6 corolla sr, but recently i b*llocksed my exhaust, in such a big way that whole manifold may need replacing.
My query is, why did this cause my MPG to drop to below half of what it was??
I dont get how the manner in which fumes from burnt petrol exit the engine influences the fuel consumption??

Any info much appreciated??

Bumpstop

I'll have a crack at this.
Someone who knows can correct me.

In a perfect (ideal) engine the exhaust valve fully closes at the top of the piston exhaust stroke, and the inlet valve opens just as the piston goes down on the intake stroke.

In reality air is a slow moving fluid, so at high rpm the two valves are practicaly open together as the piston reaches tdc at the exhaust/ intake stroke. The exiting exhaust gases are used to suction draw in(syphon), a little more air fuel mixture. the exhaust back pressure ( the restriction of the pipe ) slows down the escaping gases and this time delay allows the slow moving gases to suck more fuel air mix in.( or at least block the petrol /air charge from exiting throught the open exhaust valve)


A leak in the exhaust system allows the gases to leave the chamber quickly, and no, or less back pressure is generated. this actually allows some of the inducted petrol/ air charge to be blown out with the exhaust gases. so this portion is wasted and you have less power and use more petrol.

Normaly the intake valve may even stay open a little as the piston goes up on the compression stroke, The petrol /air charge like a fluid, has inertia, and this enertia will force a little more in, before the compression forces overcome it.( though this may be the realm of a racing camshaft, and would explain why race engines don't idle well)



I hope this makes sense, I probably explained it badly, but you get the idea.

The oxygen sensor just downstream from exhaust manifold senses lean condition due to leaky manifold and requests the ECU to dial in more fuel.

s-cogan

Bumpstop wrote: »

I'll have a crack at this.
Someone who knows can correct me.

In a perfect (ideal) engine the exhaust valve fully closes at the top of the piston exhaust stroke, and the inlet valve opens just as the piston goes down on the intake stroke.

In reality air is a slow moving fluid, so at high rpm the two valves are practicaly open together as the piston reaches tdc at the exhaust/ intake stroke. The exiting exhaust gases are used to suction draw in(syphon), a little more air fuel mixture. the exhaust back pressure ( the restriction of the pipe ) slows down the escaping gases and this time delay allows the slow moving gases to suck more fuel air mix in.( or at least block the petrol /air charge from exiting throught the open exhaust valve)


A leak in the exhaust system allows the gases to leave the chamber quickly, and no, or less back pressure is generated. this actually allows some of the inducted petrol/ air charge to be blown out with the exhaust gases. so this portion is wasted and you have less power and use more petrol.

Normaly the intake valve may even stay open a little as the piston goes up on the compression stroke, The petrol /air charge like a fluid, has inertia, and this enertia will force a little more in, before the compression forces overcome it.( though this may be the realm of a racing camshaft, and would explain why race engines don't idle well)



I hope this makes sense, I probably explained it badly, but you get the idea.

this was all i needed(understood), explained both my consumption and power problems.
thanks alot, lads.

CiniO

Bumpstop wrote: »

I'll have a crack at this.
Someone who knows can correct me.

In a perfect (ideal) engine the exhaust valve fully closes at the top of the piston exhaust stroke, and the inlet valve opens just as the piston goes down on the intake stroke.

In reality air is a slow moving fluid, so at high rpm the two valves are practicaly open together as the piston reaches tdc at the exhaust/ intake stroke. The exiting exhaust gases are used to suction draw in(syphon), a little more air fuel mixture. the exhaust back pressure ( the restriction of the pipe ) slows down the escaping gases and this time delay allows the slow moving gases to suck more fuel air mix in.( or at least block the petrol /air charge from exiting throught the open exhaust valve)


A leak in the exhaust system allows the gases to leave the chamber quickly, and no, or less back pressure is generated. this actually allows some of the inducted petrol/ air charge to be blown out with the exhaust gases. so this portion is wasted and you have less power and use more petrol.

Normaly the intake valve may even stay open a little as the piston goes up on the compression stroke, The petrol /air charge like a fluid, has inertia, and this enertia will force a little more in, before the compression forces overcome it.( though this may be the realm of a racing camshaft, and would explain why race engines don't idle well)



I hope this makes sense, I probably explained it badly, but you get the idea.

Lot's of science in it, but little true.
It has very little impact on fuel consumption.

The real impact on fuel consumption is that in all modern cars, there are lambda sensors located in exhaust. Usually one in front cathalytic converter, and one behind. These sensors, analyse the exhause fumes, and check if fuel-air mixture was too rich in fuel or otherwise.
So readings from lambda sensors have major impact on ECU which is deciding how much fuel injectors have to put into every cycle.
If exhaust is not tight, it might falsify readings from lambda sensors, and make ECU to put way more fuel then needen, causin dramatic rise in fuel consumption. There isn't really any bigger philosophy in it.

Bumpstop

The effects of a bad exhaust leak causing lack of power and poor fuel consumption, were around long before Lambda sensors and car electronics were invented. Back in the days of carbs this problem was just as prevelant.

anything that would cause the lambda senor to call for large amount of extra fuel to be added to the mixture, in contradiction to other sensors around the car, maf, Crank sensor, knock sensors, etc, would cause the ecu to go into open loop mode and it would ignore the lambda readings as out of range.

The open loop mode would cause a decrease in mpg but it would be hardly noticeable. As the ecu has a host of engine monitoring devices around the car it does not soley use one sensor to determine the lenght of the injector opening time.

SouperComputer

Bumpstop wrote: »

anything that would cause the lambda senor to call for large amount of extra fuel to be added to the mixture, in contradiction to other sensors around the car, maf, Crank sensor, knock sensors, etc, would cause the ecu to go into open loop mode and it would ignore the lambda readings as out of range.

That depends on the the size of the leak. It can still compensate with the fuel trim, use a lot more fuel than usual (definitely noticeable) and not throw an OBD code or go into open loop.

OP as pointed out, the main cause of the excessive fuel consumption is most likely the fact that the O2 sensor is reading lean becuase of the leak, as a result the ECU is compensating with more fuel. Also colder weather may play a factor too. Thirdly, the richer mixture is probably fouling the plugs, affecting their peformance. The affects of this far outweigh the affects of the change in the backpressure in the exhaust system.

Bumpstop

In my era broken exhaust manifolds were an all to common problem, this always effected power and fuel consumption, as these cars had no electronics, the o2 sensor can be pretty much ruled out, though I accept it would have a small effect.

Valve overlap on a petrol engine at high revs is unavoidable, and the exhuast gases keep moving after the piston reaches tdc. Back pressure was a bad word I used. I should have said, this inertia is used to scavange air out of the cylinder. The manifold acts a guide and a syphon, a leak here and the syphon effect is lost.

I didn't just figure this out myself, I wish I did. This type of fluid motion is the basis behind modern vvti engines. These type of engines maximise the efficiancy of the valve overlap times by adjusting the rotational position of the camshaft. Indeed dual vvti is getting popular now, where both intake and exhaust cams have vvti fitted.

This effect will only really be seen on a leak very near the manifold, the op did say his manifold needed replacing, so I assume it was damaged , a leak further down the system would not have such a dramatic effect, and the o2 sensor would probaly come more into play in that case.

lil-dude-01

Symtoms sound simelar to problems iv been having with my corolla!!!
I hav a 1.6 corolla sr 2000 and the engine management light will not go off?? now my father knows a bit about cars and he doesnt know what to do but ether does my mechanic!! the car has less power than a 1.3 and drinks petrol and as a student its getting very tireing!!! got my mechanic to plug the ECU in to try locate the problem and Bank one came up as the issue so.....
1st. Changed the front lambda sensor, light went off for 2 days then came back on!
2nd. put the old lambda sensor from the front into the back, light went off for 2 days then came back on!
3rd. got it plugged back in and Bank one came up again! so we changed the airflow meter/sensor.
4th. plugged in again SAME responce!!!!!
WHAT DO I DO??? could a Knock sensor be the problem?? HELP!!!!!

TestTransmission

lil-dude-01 wrote: »

Symtoms sound simelar to problems iv been having with my corolla!!!
I hav a 1.6 corolla sr 2000 and the engine management light will not go off?? now my father knows a bit about cars and he doesnt know what to do but ether does my mechanic!! the car has less power than a 1.3 and drinks petrol and as a student its getting very tireing!!! got my mechanic to plug the ECU in to try locate the problem and Bank one came up as the issue so.....
1st. Changed the front lambda sensor, light went off for 2 days then came back on!
2nd. put the old lambda sensor from the front into the back, light went off for 2 days then came back on!
3rd. got it plugged back in and Bank one came up again! so we changed the airflow meter/sensor.
4th. plugged in again SAME responce!!!!!
WHAT DO I DO??? could a Knock sensor be the problem?? HELP!!!!!

Start a new thread please