# Does an engine run wide open use same amount of fuel at light load and at full load?

Ok, heres a question, Its a question that I have genuinely wondered about, and I hope it doesn’t aggravate anyone thats not the intention.

If I have a car that has a carburetor and is a stickshift with a 4th gear that is direct drive.

We put the car on a dyno for one hour, run it wide open flat out in 4th gear for one hour and its running at 5000rpms, lets say it used 1 gallon of fuel in that hour.

Same car on the dyno run wide open in 4th gear, dyno puts a load on simulating a 25% grade, engine only turns 4000rpms. Would it still use the exact same amount of fuel?

I say YES

Would a fuel injected engine have different results?

I say possibly.

Why are you asking this silly question Rick???

Confession time. Its not about a car but it could be.

I am actually wondering about my mantis tiller, I run it wide open in the dirt and the rpms vary with the type of dirt, moisture in dirt ect…, there is no governor to open the throttle more to maintain RPM because the throttle is already wide open. So if the throttle is constant, but the load varies, the amount of work being done is the same so it uses same amount of fuel right?

I am not saying im right, I just want to better understand engines. I very well may be wrong… I just don’t know…

There are too many details left out of the equation @Rick. What kept the engines rpm at 5,000? Does “flat out” indicate that the throttle was wide open in both runs? If the engine’s peak torque is near 4,000 rpm fuel usage may improve when loading down to that speed.

The tiller does have a governor. Setting the throttle at wide open on the tiller sets the upper limit of the governor at wide open but with no load the engine speed reaches the governed rpm and the throttle is closed enough to drop below the governed speed where it will oscillate from above to below the governed limit. There is no direct connection from the throttle lever to the carburetor throttle butterfly.

Good question. My guess is that with your Mantis tiller, the amount of fuel would be the same. At wide open throttle, the maximum fuel/air mixture is reaching the engine. When you are tilling the soil, at wide open throttle, the engine slows down, but with the throttle wide open, you are using the same amount of fuel regardless of the load.

The tiller does have a governor. Setting the throttle at wide open on the tiller sets the upper limit of the governor at wide open but with no load the engine speed reaches the governed rpm and the throttle is closed enough to drop below the governed speed where it will oscillate from above to below the governed limit. There is no direct connection from the throttle lever to the carburetor throttle butterfly.

My big tiller has a governor, I didn’t realize the mantis did, it bogs down some even in light duty conditions.

I guess my question is Null, lets say that an engine has a fixed throttle position, no governor, the more load you put on engine the slower the rpms are. Will they use a fixed amount of fuel?

Im not sure, do weed whackers have governors? I didn’t think they did. Lets say its a weed whacker engine, at throttle wide open, no load its turning 8k rpms, wide open with load its going 7k rpms.

My answer was based on the assumption that the Mantis tiller does not have a governor. I am now really curious. I have an Earthquake 2 stroke tiller with a throttle control. I assumed that it also has a governor. I will go out and check it.
The hand throttle control doesn’t mean the engine does not have a governor. Both of my lawnmowers have handle throttle controls and both have governors. One mower has bad rings and when it is under load it really puffs the blue smoke.

To answer your car on the dyno question, probably not. Fuel flow is not just a function of rpm, it is a function of power output and brake specific fuel consumption (BFSC) at that particular rpm. BFSC is essentially a measure of efficiency.

You can’t simulate a 25% grade at 4000 rpm on a dyno and compare it to wide open throttle at 5000 rpm. WOT at 5000 rpm, steady state, means the engine is loaded by the dyno until it reaches steady state. The dyno would be measuring the torque output at a steady 5000 rpm. Horsepower would be 5000rpm times the torque ft-lbs divided by 5252. The same is true running 4000 rpm at WOT,steady state. The dyno is measuring torque at a steady 4000 rpm. The power output will be 4000 rpm x Torque at 4k / 5252. The BFSC will be different, too. If peak torque is produced at 4000 rpm, that will be close to the most efficient BFSC. At 5000 rpm, you will produce more horsepower and less torque but the BFSC will not be at the most efficient point. You will use more fuel because you are producing more horsepower and use more fuel because you will be at a less efficient point.

A carb vs fuel injection at WOT will produce essentially no difference in horsepower. There are, however a bunch of cool things you can do with fuel injection to MAKE more horsepower AND fuel injection is more efficient at part throttle. Plus with direct injection you can inject gas directly into the combustion chamber to make even more horsepower and efficiency.

Maybe I’m just missing it but what a throttle does is open and close the fuel valve. If the throttle is wide open, the max amount of fuel is being delivered regardless of the rpm. So if you have it bogged down so that rpm is less, the same amount of fuel is being delivered to the cylinders. That’s what throttles do. But don’t go running an engine at 5000 rpm no load and same thing for a tiller. They have governors on them to prevent blowing the engines up.

An engine running at a given RPM with the throttle plate completely open is more efficient than one with the throttle plate partially closed. This is part of the reason manufacturers are switching to direct-injected engines, as they take the throttle plate out of the equation.

If you take two engines that have throttle plates and run them with the throttle plates in the same position, whether throttled back or wide open, the one running at a higher RPM should be using more fuel, assuming the carburetor or computer control system is keeping the mixture correct.

The throttle of a gasoline spark ignited engine controls the air flow. The fuel metering device(s) whether carburator or injectors meter the fuel to provide the correct mixture. The engine is basically an air pump. The more air it can process the more power it can produce.

In the orignal proposed situation of the engine working at 5000 RPM versus 4000 RPM at full throttle the former situation would consume more fuel per hour than the later. This is assuming that the volumetric effeciency remains the same i.e. VE ~ 95% of swept volume.

Here is an interesting puzzler. A 392 hemi with two four barrel carburators on stock manifold is driving a flat bottom ski boat. The throttle pedal is on the floor. The vacuum secondary plates are only slightly open. When the secondary plates are manually openned, the flat bottom does not gain or lose speed. Is there a problem with the carburators?

It would be nice if weed wackers had governors but the ones I have worked on and used did not. The throttle control is connected directly to the carburator throttle. If you open the throttle, without load, the engine takes in all the air it can handle and races. If you load the head before you open the throttle, the speed will stay low because of the head load i.e the air flow is limited by the speed of the engine.The one problem I have with weed wackers is the inability to maintain a constant speed when edging. The engine races when the cut is light and lugs when the string digs into the grass.

It’s a good question. Forget the dyno, say on level ground w/the throttle wide open the rpm is 5000. There’s several forces acting against the engine, the wind resistance is probably the major one. Then you go up a hill, no change to the throttle, rpm drops to 4000. Now the major force acting against the engine is probably the hill; i.e. the force needed to push the mass of the car up the hill. The flow of gas in either case is directly proportional to air flow rate into the engine. So if the air flow is the same, the fuel flow is the same. Air flow is proportional to the throttle opening and the intake manifold vacuum. There’s no change to the throttle opening, so to answer this question you’d need to know if there was a change to the intake manifold vacuum when going up the hill. I don’t know whether there would be or not.

In this silly exercise, view the engine as an air pump. The faster the pump runs, the more air it pumps. If you are mixing gasoline with this air, the fuel consumption will increase too…The engine running at 5000 RPM will register a higher power output than the engine running at 4000 rpm. More power, more fuel…

Gasoline is your energy source. The engine will use the amount of gas necessary to do the required work, whether it’s carbureted or fuel injected. Any more and it would flood. Less load means less gas used. Therefore, an engine running with no load on it at a given speed will not use as much gas as an engine running the same speed with a load to pull.

The details as to how the engine’s fuel metering systems accomplish t his can be complicated. But the bottom line is that they use less gas without a load than they do with a load.

I always approach these questions using limits. Start wide open at 5000 rpm on a dyno and measure fuel flow. Now, still wide open, brake the dyno down to 1000 rpm. I have to believe the fuel flow would be much less. Therefore, at 4000 rpm it would also be less.

The difference between the load and no-load situation is the effect that manifold vacuum has on it so mileage should drop with a load on the engine.

Some may remember back in the 70s that some GM cars had a factory vacuum gauge in the dashboard and it had a multi-colored bar instead of numbers on it.
The object of the exercise was to try and keep the needle in the green for better economy instead of the yellow or red portion of the bar by developing a feel for the pedal.

Here is an interesting puzzler. A 392 hemi with two four barrel carburators on stock manifold is driving a flat bottom ski boat. The throttle pedal is on the floor. The vacuum secondary plates are only slightly open. When the secondary plates are manually openned, the flat bottom does not gain or lose speed. Is there a problem with the carburators?

Okay, I’ll bite. The carb is functioning properly, although the engine is a bit over-carbed. The carb flows more CFM than the rest of the intake–something other than the carb is the “weak link in the chain.” Even with the secondaries partway, it still flows as much as the rest of the intake, so opening them further makes no more power–although it might enrich the mixture a bit.

(Let me know if that’s right…my carb experience is limited to small-engine stuff…and CV units on a few Honda motorcycles I’ve owned.)

A couple of thoughts:

Engines running wide open are producing the maximum amount of horsepower for the rpm.

So if I am running an engine at 4K vs 5K, then more fuel is going to be used at 5K - assuming 5K isn’t above where the carb flow maxes out, nor above where the engine becomes more inefficient due to pumping losses. You can verify this by looking at the horsepower vs rpm curve.

A different way of looking at this is that engines are most efficient when running at wide open throttle (WOT), and fuel flow is sort of proportional to engine speed. So for best fuel economy, you want the engine as close to WOT all the time - and there isn’t a very practical way to make this work without causing other issues.

To me, the only real question is how much fuel is used for the work being done. We had posters at one time or another say that even though their car used more gas and got fewer miles per gallon going faster, they could save gas on a trip by driving faster and spending less time on the road. That kind of logic boggles my mind and some of us just need to simplify things.

My newer tractor uses a hydrostatic transmission which by it’s nature uses more gas per hour then a gear driven variant I had before. The difference is, it does literally, twice the work in some situations making it more efficient when compared to “work done” .

So talking about efficiency on a dynamometer does little for me compared to actual distance traveled or work done for a given amount of fuel used and when the perspective switches to @GeaogeSanJose point of view, it makes a little more understandable for me.

As @CapriRacer and @dagosa noted, an internal combustion engine runs at peak efficiency at full wide open throttle. It’s least efficient when operating at partial throttle. That is due to both pumping losses at partial throttle, and more importantly, the less dense mixture available at partial throttle. Efficiency is defined as the amount of work the engine does per unit of fuel consumed.

Of course, “best engine operating efficiency” and “best gas mileage” are very different. Most are only concerned with the latter.

A different way of looking at this is that engines are most efficient when running at wide open throttle (WOT), and fuel flow is sort of proportional to engine speed. So for best fuel economy, you want the engine as close to WOT all the time - and there isn't a very practical way to make this work without causing other issues.

Yes, but…most enigines (both EFI and carb) have enrichment circuits that engage as you near WOT (occasionally, they only engage at WOT+some minimum RPM). So practically, best BSFC will be just before enrichment begins…80% of WOT, perhaps?

There have been no governors on any 2 stroke yard equipment that I have worked on. And if the tiller is 2 stroke it is unlikely to have one.

As for the boat secondaries, @researcher, the secondaries not fully opening at continuously wide open throttle would indicate that the carburetor capacity is too great for the engine.