Selective number of active cylinders - fuel economy

Well, I'm brand new to the group, so I don't know if this topic has been covered before or not. Since it seems somewhat obvious, I assume that it has. Forgive me in advance if that is the case, but my curiosity compels me to ask anyway.

First of all, I am NOT an engineer, but I have had a fair amount of experience in the automotive arena.

Here is the question: With a multi-port fuel injection system, why can't you selectively cut back the number of "active" cylinders, i.e. shut the power off to one or more (depending upon the number of cylinders in the engine) of the injectors during light or almost no load conditions? How much horsepower does it actually take for a Honda Civic (just as an example) to cruise at 55-60 on a four lane? If you cut back to 2-3 cylinders, wouldn't that save fuel? And couldn't you run on 1-2 cylinders at stop at idle?

I know there are a lot of other dynamics in the equation, such as how the EEC would react, the "active" cylinders would have to do comparatively more work, what the O2 sensor would think, etc. I also realize it would induce an imbalance in the engine, but at low or almost no load, the imbalance (vibration) would be minimal.

Since the cylinder would only be pumping air (isn't that what an engine is at the most fundamental level?), it shouldn't be chemically harmful. I also realize that, through mechanical/friction losses, the mileage gain wouldn't exactly be 100%, but it should help at least a LITTLE!

It also seems like it could be automated and customized for more power vs more economy, shut off for reduced traction situations, etc. Between intake vacuum, throttle position and speed, it could be virtually transparent to the driver.

I know Cadillac and maybe others made something on this order back in the 80's, but it used selectable valve engagement, versus selective fuel injectors.

How about it? Any thoughts?

Welcome to the community!!!

Without combustion happening to aid ring seal, you may get a lot of oil in that cylinder. Once it starts back up you might have trouble with emissions from oil burning in that cylinder. Revisions to ring design and PCV design might be necessary.

Without combustion but with the air still coming in, you are not only asking the other cylinders to run the engine, but there is still a compression stroke on the uncharged cylinder. That is the main reason why if you pull a plug wire on a 4-cylinder engine (or even an 8) it runs so pathetically or barely runs at all. Unless you disable the volume of that cylinder by deactivating the valves, you'll counteract any fuel savings by having to supply the other cylinders with more air and fuel to compensate for the work its not doing.

Basically, the engine takes lets say 16 hp to maintain idle. If you drop two of the four cylinders injector pulse, you've reduced its power capacity to 8 hp, but the fact that there are still four compression strokes means it still requires 16 and it would stall. What you would have to do is have the computer open up the throttle a bit and supply the two running cylinders with more fuel. Since you're now still making 16 hp to support idle, you've not saved any fuel, and in fact may have lost some. If you disable the valves, too, you're reducing the hp requirement to keep the engine running. Then the other two cylinders are only supporting their own compression, plus the minimal friction of the other two.

Caddy's 8-6-4 was a pretty good idea, but in the end it didn't save much fuel at all. The cylinders are dropped during periods of lowest load. Since fuel consumption is at its lowest during those times anyway, the amount of fuel that is saved then made little impact on the overall MPGs.

I think you would get a better increase if you recurved your ignition and fuel. Most factory ECMs keep the car a tad rich to prevent detonation and to keep NOx emissions in check. Dropping a little fuel and making sure the ignition curve is as advanced as it can be without detonation will pick up a couple MPGs on most cars. In the process, you'll also pick up some power.

Or convert to a diesel :)

i don't believe this was discused in the past but chrysler has succeeded in making a system like this, i don't know exactly how they shuft them off, they might keep the exhaust valve open and shut off the fuel or something but they use it in some of their 5.7L hemis with some good success and apparently you dont netice the cylenders shutting off and turning on. the biggest system ive seen for something like this was on the jeep hurricane, a concespt vehicle with one 5.7L V8 hemi in the front and one in the back. with this vehicle you could run on 4,8,12 or 16 cylenders so you'd have one engine running on half its cylenders and the other one not running at all when you're cruising but if you punched the gas you'd have all 16 running, getting probably aroun1 mpg or something lol. and welcome to the af community, my suggestions for people is just to be respectful, use the search button before starting a thread, and don't resurect old threads.

I'v heard of this before, almost swear I'v read about it on production (or upcoming production) stuff. I know my friends f150 (4.2 (I think) v6) will shut off three cylinders if it starts to overheat.

it is on production stuff, the dodge hemis use it, check out the website i think its in the 300