I was thinking about EFi today and how it's a shame when I'm driving down the motorway at 70mph that my engine is going 5k and I'm
consequently getting jz MPG.... It seems a shame to be producing 60-70Bhp when to keep going at a constant speed I actually need about 7!!!
So.... what would be the downsides of having it so that the ECU switches off spark and fuel to one cylinder during certain conditions. You
wouldn't wash the bores (and still waste fuel) like if you just chopped the ignition... The piston would still get cooled by the oil from
underneath and it wouldn't be getting so hot without ignition events anyway. It would confuse the hell out of a lamda sensor so not good for cars
on closed loop....
If you wanted to get really trick you could rotate the "dead" cylinder around the block so that you don't one end of the block being
cooler than another bit...
Genius idea or seriously flawed?!?!? Might sound a bit horrible, especially if the engine decided you only needed one cylinder!!!
big mercs have been doing this for a while, shutting down 1/2 the engine on v8s etc when cruising. i guess it shows with correct development & implimentaion, its 100% safe and doable
Aha! Didn't know if was a leccy system or having a clutch to split the block into two....
have been thinking about this myself. think cadillac use it too, was on top gear a few years back, goes about mostly on 4 cylinders, and then all 8 when it has to. i just can't get my head round how it would work, especially the whole thing of rotating the non working cylinders. i guess you only need to switch off the injectors? sparks can still fire i suppose.
I suppose you could keep the spark going, yes....
Pretty simple then. I suppose it's like a soft cut on an ECU. Trouble is it would make it sound like you were driving down the road on the soft
cut and unless you've got something like a big V8/V12 it would sound a bit pants.... (a bit stuttery and you wouldn't have enough cylinders
to smooth out the sound)....
the really clever bit would be to open the inlet valves on the pistons not firing so they are not fighting with compression when its not needed. The engine would be a lot easier to turn over??
cadillac use this system when the engine overheats making it work as an aircooled engine
I think Alfa experimented with a system that kept the valves shut to reduce the puming losses, if the valves are still operating it would use a lot of
power compressing air in the idle cylinders.
Paul
quote:
Originally posted by Antnicuk
the really clever bit would be to open the inlet valves on the pistons not firing so they are not fighting with compression when its not needed. The engine would be a lot easier to turn over??
I was thinking also that the fuel saved from not supplying the cylinder would be offset by the pumping losses. Though I am not entirely sure because
you are expending similar energy compressing gas regardless of whether there is fuel vapour present??
To really make it worth while you would need to do something with the valves to prevent compression losses. On a dead cylinder you could leave the
inlet open all the time (assuming it was not a plenum inlet)
and just close the exhaust valve on the inlet stroke.
Be interesting to know how they do it for real
If you're producing 60-70 bhp and only need 7, where do the other 63+ go, or do you really need more than 7 bhp to do 70mph?
My guess, for what it's worth, is that you actually need 60-70bhp to keep going at 70mph. The big engines mentioned are effectively shutting down
to a 4 cylinder 70bhp engine from something much bigger and more powerfull
a fuel injected engine only uses as much fuel as it needs to. the amount of fuel is based on the amount of air present. which is down to how much air
is allowed in through the throttle body. cruising along at 70 in most cars the throttle isnt open that much, therefore not much air, and not much
fuel. accelerate (open throttle) equals more air and more fuel. (poorer mpg's).
does that make sense?..... i dont think ive explained it that well...
back to original question, i dont think closing the valves and turning off spark and fuel would make that much difference without some serious
research and testing....
will
i think it wont work because you use what you produce else the other 2 pistons need to work harder
Aparently, citroen saxo's switch off the fuel injector to two cylinders when idling to save fuel. I have also been trying to think of a way to do
this but maybe have it switchable from the dashboard. Just think how much easier SVA/IVA would be if the noise test could be done on just two (or
three) cylinders
To maintain speed you balance the forward force (from the engine) with the backward force (from air resistance/friction).
You regulate the throttle so that the two forces are equal and thus constant speed. Instead of shutting down one cyclinder all the cyclinders just
work at a lower efficiency.
dinosaurjuice's explaination is my understanding too.
[Edited on 14/2/09 by Project7]
I would think would be possible to bo but ideally you would want to no fire on one cyclinder for a small amount of time.. say 20 mins and then turn
off a different one... so the engine would wear evenly....
also i had to drive my melos on only 3 cyclinders when i kept back firing on one cylinder.. ran fine !!!! but rather than leaving the valves open you
could have a one way evectric valve on the spark plug inlet that would let pressure out but be stonge enough to not give up when the engine is running
normally !!
[Edited on 14/2/09 by yellow melos]
Surely on larger engines (V8's etc) they have enough torque that the remaining cylinders are able to carry the extra load without putting undue
stress on the bottom of the engine.
The bigger engines also have more rotating mass - a bit like the difference between a heavy and light flywheel. A small engine may actually need more
fuelling to the remaining cylinders to keep the same road speed.
You could use a Throttle Position Sensor to turn on/off the injectors so that after your ignition has cut the cylnders, when you put your foot down
you have the response that you need.
a very interesting project for a V8.
If you want torque and 2500rpm cruising try a diesel (putting on crash helmet and hiding!!) and get 150bhp and 50 mpg
Steve
quote:
Originally posted by BenB
I was thinking about EFi today and how it's a shame when I'm driving down the motorway at 70mph that my engine is going 5k and I'm consequently getting jz MPG.... It seems a shame to be producing 60-70Bhp when to keep going at a constant speed I actually need about 7!!!
Possibly, but a higher final drive would reduce acceleration. What I'm suggesting here would give best of both worlds 
My maths works on the principle that when you're blatting along @ 5k + 70mph the engine isn't making 70Bhp but it is working very
inefficiently. Just using a few cylinders more efficiently would make much the same power but with less petrol wasted.....
I might investigate making something like this that acts between the ECU and the fuel injectors...
As I see it you open the throttle only enough to produce sufficient power to maintain the speed so at normal cruising speeds the engine will run at a
fairly high manifold vacuum - this leads to efficiency losses and this is where cutting fuel and spark to certain cylinders will be beneficial because
to make the power you need you will need to open the throttle more reducing the vacuum and hence pumping losses. You will still pay the price of
friction losses in the engine that are principally rev related and of cours aerodynamic and wheel, tyre and drivetrain friction losses.
The negatives are an uneven firing pattern leading to bad engine balance leading to possible crankshaft failure if the crank is performing close to
it's design parameters, reduced efficiency of exhaust scavenging and the friction, oiling up and wear consequences of running a cylinder too
cool, so you should only skip 1 in two or four firing cycles in each cylinder at a time to keep it warm.
I think that it will be very difficult to make the sort of savings you seek without risking damage to the engine - don't forget that the
manufacturers design the engine for this type of use by revising balance mechanisms and safety factors to suit. The only benifit is a possible 25 to
50% reduction in pumping losses which are in any case a lot smaller than the friction losses in the engine. I would guess that you might gain 5%
reduction in consumption with the risk of destroying a couple of engines whilst you experiment.
You could gain a lot more looking at gearing, aerodynamic improvements, low friction tyres, driving style adaption, good tuning and weight reduction
of the car and if necessary yourself 
and of course driving 5 mph slower.
quote:
Originally posted by Antnicuk
the really clever bit would be to open the inlet valves on the pistons not firing so they are not fighting with compression when its not needed. The engine would be a lot easier to turn over??
Ivan is spot on - this is all about using fewer cylinders under higher load (where they operate more efficiently) to produce a given power.
Doesn't make a huge difference because as has been pointed out you're only ever making as much power as you need, however many cylinders are
making it.
Latest generation chevy LS series V8s also do this to save a bit of fuel. Probably wont make for very smooth running if you only have four cylinders
to start with.
Liam
I could get my megasquirted MX-5 to run at an air:fuel ratio of 16.5-17:1 on cruise without it spluttering, an increase of 10% in fuel efficiency over
the std ECU, which would run at 14.7:1. If you are on carbs, I suspect you would save even more moving to EFi.
In your case though 5k rpm is quite high, lengthening the gears would have a large effect I would have thought.
quote:
Originally posted by BenB
Possibly, but a higher final drive would reduce acceleration.
quote:
What I'm suggesting here would give best of both worlds
My maths works on the principle that when you're blatting along @ 5k + 70mph the engine isn't making 70Bhp but it is working very inefficiently. Just using a few cylinders more efficiently would make much the same power but with less petrol wasted.....
I might investigate making something like this that acts between the ECU and the fuel injectors...
the 2009 dodge ramdoes this its runs the v8 up to cruise speed then switches over to 4 pots have alook at
the dodge web site.