Crazy Idea for Experimental engine...
 

Someone at work was asking me about forced induction. I explained how turbochargers and superchargers worked. I explained about the major types of supercharges (centrifugal, roots, and screw-type).

"So a supercharger is like an air compressor?" she asked.

"Kind of, except that most air compressors use piston-type compressors."

"Couldn't you use that as a supercharger?"

"I suppose you could, but there is probably a good reason that it is not."

Now, my idea is not a piston-type supercharger...at least not completely...

I was thinking about my truck's engine. It is a V8 that shuts down 4 cylinders to conserve gas.

See where I'm going here?

Would it be possible to use the non-combustion cylinders to supercharge the combustion cylinders? An engine that can run as a V8 or supercharged V4. I realize this would take some elaborate piping. The exhaust on the super-cylinders would have to be routed to the intake of the combustion cylinders when the engine is running in V4 mode, and to the exhaust collectors when in V8 mode.

Thoughts?

You also have to remember that compressing air in the non used cylinders would increase the work on the 4 working cylinders.

That's the thing, the engine only cuts out spark and fuel--it's already compressing air when the unused cylinders aren't firing.

How many cylinders are in compression stroke at any given time while in 4 cylinder mode? Are they each compressing separately so you have 4 sequential pumps or do you have 2 pumps in parallel every second stroke?

Well, if it is a cross-plane V8, that means that each cylinder is compressing in sequence. I'd expect a flat-plane V8 (where to cylinders use the same lobe on the crank) to be compressing in parallel.

Are you sure the valves on the 4 unused cylinders aren't held open so the motor won't have to be doing this extra work on compressing air then sending it out the exhaust pipe.

If they are not open but operating normaly, I see a major problem. When the intake valve opens it is drawing air in at atmosperic pressure & an ambient temp of lets say 80 deg F or about 28 deg C. The air temp & coolant temp of 190 F will be trying to reach equalbrium & the rate of heat ransfer will determine how hot the air is before it is compressed. Much of the work of mechanical energy of the compression stroke is turned to heat energy. Lets say you have 6 to 1 effective compression. Don't have a chart in front of me but lets say that raises the air temp to 500 F or 270 C. The coolant is cooling it down and as the pistons goes back down releasing pressure the coolant is trying to get the air to 190 F. The pressure gets down close to atmosheric & the air is hotter from the coolant as it is pushed out the exhaust valves to super charge the other cylinders.

You end up with no more oxygen to burn fuel but it is a lot hotter which can cause detonation.

I believe the valves are deactivated to reduce pumping loss on the engine.


Quoted from http://www.superchevy.com/technical/engines_drivetrain/accessories_electronics/0405sc_gmdod/index.html


"In order to eliminate the pumping losses," says Meagher, "you need to disable both the intake and exhaust valve." This results in a completely sealed, deactivated cylinder, which is essentially an air spring being acted upon by a piston. Virtually all the work put into it during compression is returned to the crank during decompression, finally giving credence to the old joke about piston-return springs. (That's nothing. Wait 'til you hear about the muffler valve...)

"Currently, we could disable just the fuel delivery," says Meagher, "but the valves would still be opening and closing and each cylinder would still be doing work pumping air in and out. So there would be no net gain in efficiency--you wouldn't have eliminated the pumping losses at all."

I can't see an issue with the air getting warmer because every compressor deals with this whether it is a supercharger (pick your type), turbo, or a standard air compressor. Obviously once you exceed a certain temperature it would loose its benefit but I don't see the air gaining that much temperature when the car is cruising at 2000 rpms (of course I haven't taken the time to figure out how long the air will remain in the cylinder, rate of heat transfer, etc.). You would have to have the computer automatically retard the timing ,increase the fuel injector pulse widths, and you may even have to use some type of intercooler to help shed some of that heat.

LOL...the E-Ram returns...

err...ahh.."2-stroke-ram"

There is no benefit, no more oxygen than the firing cylinders would get anyway. Flowing through the other cylinders ports reduces effiency by the restriction giving less oxygen & the oxygen it did get would be at a higher temp. I guess you could add an intercooler, but this would reduce effiency as well. Increasing the injector pulse widths would defeat the purpose, the extra fuel would have no more oxygen to burn & would just help with cooling, not power or mileage

Back in the 70s I read & saw pictures of a set up Crower Cams came up with to improve the milleage of a big block Chevy. Rocker arms were taken off on 4 cyl so the valves wouldn't open. Kind of oblong slugs instead of round pistons replaced the pistons so that the motor would not have to do the work of compressing air. I kind of thought a junkyard 283 with rings, inserts & valve job would be more cost effective, because of the big blocks recipcrocating & rotating mass.

You'd need one way valves on the air lines coming off the cylinder, otherwise it would loose the pressure on the exhaust side. Also, you'd need to be able to cut it off when the cylinders fire up again.

The compressor needs to stuff a greater volume of air/fuel into the combusting cylinders to increase power. So the compressing cylinders would have to pump every rotation. The cams would need double lobes.

If you get it to work, you will have created the heaviest supercharger in the world ;)

I see what you are saying (I guess I'm slow tonight). So really the only way for the engine generate any boost (more air pumped than the other cylinders) would be have a ten cylinder that cruised with four cylinders working or some kind of holding tank.

Any 4 stroke engine already uses a piston to compress the air, it's the basis of the whole concept...
You'de be better off just raising the compression a bit and leaving the exhaust for what it is

Older, not sure on newer commercial gas air compressors, the type you tow behind a truck use half of this theory. They are 8 cylinder but only run on 4, the other half of the 8 is where your air comes from.

Whiskyb

I thought that this was used by some diesel truck engines in the 50's and 60's - Commer, Fogden or Roots I think. I remember as a kid that they used to sound quite different.

Tim

Others have already pointed out some of the inefficiencies; I'll add that when you are down to operating on 4 cylinders, you are doing so because you don't need extra power.

Supercharging a motor has more to do with "getting more power" than it does with getting more "efficiency" out of the motor.

When your truck's motor is running on 4 cylinders, supercharging those 4 cylinders won't really increase efficiency -- and since you don't need the extra power -- why do it?

+v8

This is the answer. They are not "wasting" the air. The gain would be almost unnoticeable at all if they were. The Displacement-on-demand from GM is cool stuff. It uses the old poo-poo'ed push rods and gives better than 10% better fuel economy. That is nothing to be ashamed of on a v8.

why not just run with a tank of compressed air in the trunk, for boost when you need it.