Quick Fuelling Question...
- Thread starter rlees85
- Start date
yeah i know its weird. i mean it in a sense that the car is accelerating, not just cruising around a certian rpm.
the reason i ask is im getting high-rpm misfire, but it misfires at lower rpms in higher gears... and if the cars sucking more juice at a specified rpm in a higher gear (becuase maybe its harder work) then i cud start looking at fuelling problems...
the reason i ask is im getting high-rpm misfire, but it misfires at lower rpms in higher gears... and if the cars sucking more juice at a specified rpm in a higher gear (becuase maybe its harder work) then i cud start looking at fuelling problems...
R
ricardo_swarez
Guest
I dont think it would be using more fuel at a higher rpm, because the engine is going through the same amount of cycles weather your in a lower gear or higher, but then again I dont no if the ecu measures the torque required and adjusts the petrol and air deliver so theres a higher pressure in the cylinder for combustion, never thought of that before, good question this, will be interesting to see the responces
Deceptively easy one this
Assuming speed is constant (i.e. not accelerating or decelerating in each gear) If you are in 4th gear at 4000rpm at 50% throttle you will be using exactly the same fuel as in 3rd gear at 4000rpm at 50% throttle. The reason for this is that the engine would have exactly the same load on it, and therefore producing exactly the same amount of power.HOWEVER if you look into this more you will see that it is not possible to create both of these on the exact same bit of road, and your MPG readings would be totally different. This is where the visualisation might become tricky.
First of all in any case imagine the engine is at 4000rpm – and we will ignore wind resistance. You choose 3rd gear, and you find a road where the car maintains 4000 rpm at 50% throttle. If you were to work out torque and rpm at wheels after the drive has left the gearbox this would equal the load required to pull the car along this bit of road. Lets say it’s a hill. Now you choose 4th gear. The ratio of engine speed to wheel speed is now smaller. The result of this is 1, the engine has obviously dropped RPM and two, due to the lower ratio in gearing the is not enough torque to maintain your current speed at the 50% throttle position. Two choices, 1 increase throttle to get back to 4000 rpm at a higher speed, or two find a different bit of road (smaller hill) where you can achieve a 4000rpm at 50% throttle again. If you now find a smaller hill, where you can again reach 50% at 4000 rpm, and you work out power at the wheels this will be the same as it was in third gear. (Power = torque x rpm) However since you are now travelling faster, that is greater MPH but less load (not as steep hill) your MPG will be much better.
Make sense?
Ed
59 bhp is more than enough
Deactivated Account
I thought that although the load would be the same to your engine in each gear (disrigarding wind resistance and resistence from the ground etc) your gearbox,diff etc would transmit these rpms to different amount of turn of the wheels so therefore your mpg would be different. could be wrong though?
M
mr.grassie
Guest
your fuel consumption is all to do with the load placed on the engine.
If for example, you are accelerating hard along a strip of road in third gear, your fuel consumption may be (for example) 22mpg, but when you change into 4th the load on the engine increases, because you loose the mechanical advantage provided by the lower gear. therefore the engine must work harder. for the engine to work harder, it must use more fuel, and the economy would then drop to say, 17mpg.
This of course would level out when you lift off and stick to a certain speed.
If for example, you are accelerating hard along a strip of road in third gear, your fuel consumption may be (for example) 22mpg, but when you change into 4th the load on the engine increases, because you loose the mechanical advantage provided by the lower gear. therefore the engine must work harder. for the engine to work harder, it must use more fuel, and the economy would then drop to say, 17mpg.
This of course would level out when you lift off and stick to a certain speed.
mr.grassie said:
That is wrong, Power is torque x rpm. higher rpm = more power = more fuel used = more mechanical energy wasted getting that engine to that RPM.
If you full throttle in 3rd gear to reach Y mph from X mph over distance Z, and you this again under the same conditions in 4th from X mph to Y mph over the same distance Z and in both cases foot to the floor, although 4th gear would take longer you would get a better mpg than in 3rd, even though 3rd took a shorter time.
Ed
R
RIK
Guest
in other words(that i understand better)
going at high revs in 3rd will ALWAYS use more fuel than low revs in 4th
going at high revs in 3rd will ALWAYS use more fuel than low revs in 4th
rlees85, check ignition first. Its more likely, change rotor arm and make sure plug gap is correct. IMO its _very_ unlikely to be fuel system, however give it a service anyway. The pumps pump way more fuel than is actually needed and the rest is sent back to the tank. If there is that much of a restriction you would really know about it in other ways.
59 bhp is more than enough
Deactivated Account
yeah my point was that it doesnt matter what speed you are goin, 4000rpm is still 4000rpm no matter what velocity your travelling or what gear your in. thus same amount of strain on the engine, like you said. if you were travveling at 4000rpm in 4th you will be doing x m/s but at 4000rpm in 3rd you will be doing z m/s. x and z will be different speeds because of the gearing. thus if x was the slower speed. therefore your travelling less distance for the same amount of time and the same wear on you engine. so yes you will be using the same amount of fuel but travelling a shorter distance or not as many miles so your mpg will be different. im probably wrong i just tryin to understand thats all.
