Lawnmower Carb on V8

[Isaac Zachary]

Originally Posted by ps2fixer

Of my understanding it's to reduce pumping losses to achieve higher max air flows.

Bingo!

Reducing intake pumping losses are to improve MAX air flow. Unless you're at max throttle then it doesn't matter what the restriction is.

You can increase efficiency if you reduce pumping losses in the:

• Exhaust system
• Oil system
• Coolant system
• and Intake system

But three of those don't have much of an effect on immediate power output whereas the last one does.

Imagine if your throttle were controlled directly by the engine's own oil pressure or coolant flow. Maybe you have some sort of electric oil pump or water pump that you control its speed. You couldn't just turn down the pump to reduce pumping losses without affecting your throttle if cars were built like this.

The easiest way to reduce intake pumping losses is to floor it. That or remove the throttle valve all together.

Clogged or restrictive air cleaners don't have much of an effect on fuel economy except in two situations.

1. On older engines with carburetors since a clogged air filter can cause the engine to run richer.
2. On any engine that the air cleaner is so clogged you have to make the engine run at full throttle causing high load enrichment.

But just having a restrictive or clogged air cleaner in itself doesn't hurt fuel mileage as the throttle simple is opened slightly more to compensate.

The total air restriction has to remain the same to maintain a constant speed on a flat grade, and that's the sum of the air filter, the throttle and rest of the intake design.

• If you make it flow better but cruise at a constant speed the throttle just closes a bit more making the restriction the exact same again.
• If you make the intake manifold or air filter more restrictive the throttle just opens a bit more making the total restriction the exact same.

Intake pumping losses are not fixed with changes to the intake design unless we're talking about trying to get more power at full throttle, or a diesel engine. The biggest restriction is the throttle. If you can't keep the throttle wide open or remove it then there's no benefit from a less restrictive intake.

Put it this way, most of the time the only true restriction is the throttle. It's like having three or four good eco drivers in the car and one terrible driver. It doesn't matter how good the eco drivers are if you put the terrible driver in the driver's seat. You have to get him out of the driver's seat and put a good driver there if you want better fuel mileage.

The same with the throttle. As long as it's creating "the restriction" there's no point in opening everything else up. First design a car without a throttle and then we can discuss reducing intake pumping losses.

The only real way of reducing pumping losses in the intake is to design the engine to run with a throttle that's more open. Smaller engine, higher gearing and EGR all accomplish that. Those techniques make the throttle tend to stay more open. More open throttle means you're lowering "the restriction."

[pgfpro]

Originally Posted by Isaac Zachary

I jumped through it in parts so didn't see it all. But I do agree with him to a degree.

Carbs do have an advantage of getting fuel to atomize more due to the distance, and therefore time, that the fuel needs to evaporate in the intake. Direct injection leads to more particulate matter, a by-product of liquid fuel droplets in the combustion chamber.

But angles and turbulance actually tend to be enemies, not friends to atomization. Any liquid that spins tends to spin out of suspention and pool.

Perhaps the most ideal intake would be one that has a single straight runner into each cylinder with a single carb at the end. Also, with the intake designed more for average power, not full power. You want your intake runners as thin as possible to keep air speed up and therefore atomization. Big intakes with a central carb let the air thru slowly at cruising speeds which lets fuel fall from suspention.

The cross-plane V8 design also is more balanced than an inline 4 cylinder or a V-6. The balancing helps reduce vibrations without the need for balancing shafts. Both balancing shafts and vibrations sap away energy resulting in reduced power.

I agree with what your saying 100%

In the video he said with just the lawn mower carb the engine ran rich so he added the bi-pass valve and then it ran at stoich. I was referring to when he was just using the lawn mower carb itself it couldn't even flow enough air even at light load freeway.

All the things you mention is what I use on my setup.
Lean burn 20:1 to 30:1 A/F depending on load.
Cold EGR
FE Cams that are adjustable
With all the above I can achieve -1.0 to +1.0 inch/hg at very light load steady state driving conditions.

[mpgmike]

It seems to me that removing the throttle to reduce inductive pumping losses would be offset by the increased compressive losses. It was much easier to get the air in, but then the engine has to work harder on the compression stroke.

[ps2fixer]

@Isaac Zachary

You make a lot of good points, I agree with basically everything you've said. Like you've mentioned, diesels don't have a throttle plate and besides the higher compression the engine is effectively the same as a gasser, but they achieve higher efficiency numbers (fuel to mechanical movement).

I've done some thinking on how to make a gas engine run like a diesel with no throttle. Pretty much best I've came up with was direct injection that injects near TDC and continues to pump fuel in for as long of the stroke to reduce the instant burn effect to keep pressures and heat down. Maybe something like a staged injector where it injects as like a pulse and have 2-4 of them per cylinder. This concept of mine would be compression ignition btw, no spark plug since you can't keep the ideal air fuel ratio with the throttle wide open unless you go the route of a hit and miss engine which probably wouldn't be too accepted as a design for a car engine. I'm no engineer but I do like to know how things work and generally can work out things in theory pretty well. I don't think I'll be building a compression ignition gas engine any time soon. Mazda has though.

https://www.roadandtrack.com/new-car...soline-engine/

Another interesting concept I've had since I got a prius with the atkinson cycle engine, what if the engine was sized to the load needed to maintain speed at say 70mph or whatever the max speed the vehicle would be traveling at, and run it at low compression with the throttle wide open. For take off, it uses higher compression for the more needed power (maybe a super charger too?). Clearly at lower speeds it wouldn't be able to work at perfect efficiency, but I'd think the effect would be pretty huge having a smaller engine running closer to ideal conditions more of the time. VVTi exists, so the prius engine design should work for that concept I'd think with the vvti on the intake valve adjusting the effective compression ratio on the fly.

Couple big questions with that concept is compression ratio efficiency vs pump losses that are reduced with the wide open throttle plate. I suspect the overall effect is nearly a wash or a company would already do something similar for a "normal" car since the concept isn't that complex and the tech already exists to pull it off.

A while back I had the idea of why not have 2 engines in a car, one for steady speed with a little overhead for small hills and such so it runs basically wide open all the time and the second engine acts sort of like the prius traction battery, it assists with take off and is the real "power" for the car to get up to speed. Clearly it would need some sort of clutch system between the two engines. Since I've seen the prius design, I wonder if the CVT design could be designed with 2 engines fairly easily. That setup would probably require the electric motors and a similar setup to the prius except doubled up and somehow tied together. I haven't thought too much on this design besides the basic clutch concept.


@mpgmike

The effective amount of air that gets into the engine is still the same even with the smaller carb for the same power output level. Air in dictates how much fuel is needed on a gas engine. The pump losses if measurable could be done with a MAP sensor and a data logger. Go say 55mph on a stretch of road and average out the vacuum reading, then repeat the same test with an extreme restricted intake to see if it makes any difference. If the air into the engine is the same, the vacuum readings should be the same for the average. I'd be interested in seeing something like this done, clearly max hp is massively effected, but cruising speeds isn't done at max hp. In my head, the peak vacuum would increase since it's harder to move the air, but the average is what an EFI computer uses to calculate the fuel charge needed to be injected to be at the right air fuel ratio.

The more I think about this, the more i think Isaac Zachary is right that the intake restriction is more or less a moot point when dealing with a gas engine. To get into HVAC systems, there's a long small copper pipe that's used to create a restriction for the liquid refrigerant to be released into the evaporator at a fixed rate. There's a different design that you can use effectively a needle valve. Both systems operate the same way if the needle valve is adjusted for the same flow. The long tube would be a highly restricted intake vs the needle valve would be the throttle plate. If there's a difference in efficiency, it's unmeasurable in ac systems from my understanding and that's with a liquid.

[Isaac Zachary]

Originally Posted by mpgmike

It seems to me that removing the throttle to reduce inductive pumping losses would be offset by the increased compressive losses. It was much easier to get the air in, but then the engine has to work harder on the compression stroke.

The idea that higher compression leads to more engine braking is not true.

Air in a cylinder with a piston acts like a spring. Even if no fuel is being burned, the amount of energy to compress the air is nearly the same amount for it to uncompress and push the piston back down. This is why a lot of modern diesel engines have terrible engine braking without the aid of something like an exhaust brake.