So is my understanding correct? In layman’s terms, the accelerator on a carby engine is mainly a device which controls airflow (rather than fuel as many people think), but it also has a secondary, specialised direct fuel control function as well. So saying the accelerator doesn’t regulate fuel is not truly correct, although for a basic explanation, it is.
Essentially yes, that’s correct.
But to give a bit of perspective, on a variable-jet carb, such as an SU or Zenith Stromberg as used on older British cars, there is no accelerator pump. Instead there’s a damper that slows down the piston and thus modulates the rush of air from tromping the throttle, allowing the fuel flow through the jet to keep pace. On fixed-jet carbs (the kind most cars use, and most people are familiar with), the problem is addressed with the accelerator pump. This gives a one-time short squirt of fuel to match the rush of air. Calling it a fuel control function, or saying it regulates fuel, is not inaccurate. I would choose to describe it in other words to avoid any possible confusion with the main fuel control mechanism.
…slows down the piston…
That’s referring to the carburetor piston, not an engine piston.
Agreed, but isn’t the airflow through the carburetor and into the intake manifold directly related to the difference in pressure between the manifold and the outside, ambient air pressure? Opening the throttle wider connects the outside air more directly to the manifold and decreases the vacuum in the manifold, so it looks to me like the transient condition would be to decrease the airflow and hence the fuel flow out of the nozzles in the carburetor venturi. Hence the need for the accelerator pump. Once the throttle is opened and the extra fuel is available the engine will speed up, if it can, reestablishing the correct relationship between outside air pressure and the vacuum in the manifold. This allows the proper flow through the carburetor venturi and thus the correct fuel air ratio, if the carburetor is in good working order.
It you open the throttle suddenly in fifth gear even at freeway speeds, well my freeway speeds anyway 
, the engine is going to protest strongly because there just isn’t enough torque available to accelerate the car and so the engine can’t speed up very fast.
Allow me to straighten out my last rather garbled attempt.
The engine takes in air equal to its own displacement every other revolution so a 200 in[sup]3[/sup] engine running at 2000 rpm takes in 200*1000 in[sup]3[/sup]/min or about 116 ft[sup]3[/sup]/min of air.
This air flows through the venturi in the carburetor and the pressure drop in the venturi, and that’s what determines the fuel flow, is the same as long as the flow rate is the same. If the tach doesn’t move then the fuel flow shouldn’t change.
All that stuff about manifold pressure is true but I think it’s irrelevant. The throttle plate, which is what is controlled by the accelerator pedal, constitutes a restriction in the air intake system. With the throttle open so as to allow the 2000 rpm there needs to be a pressure differential of a certain amount in order to produce a flow rate of 116 ft[sup]3[/sup]/min. If you open the throttle then the restriction in the air intake is less so less pressure drop across the throttle is needed to produce the same airflow. If the tachometer shows the same rpm then there is the same amount of air going throught the carburetor venturi and the fuel flow should be the same as before you opened the throttle.
Glad somebody asked this question. I started a similar thread a while ago then promptly lost internet access for a few weeks and never found out the answer!
So (slight hi-jack), in the case where i’m in a high gear say going up a hill, and needing to open half throttle to maintain speed, i would actually be better (from a fuel consumption point of view anyway) to lower a gear and use less throttle? Depite the higher revs, the lower gear uses less fuel?
Am i understanding this right?
Good question and it’s resulted in my rethinking my previous answers.
I’m sort of arguing with myself back and forth on this issue. It’s just barely possible that I analyzed this wrong (which would be a first ). It’s probable that when the throttle plate is not fully open that you can’t get enough air to have an air intake equal to the displacement of the engine. That is, that the only time you intake a volume of air equal to the displace ment of the engine every other revolution is with the throttle fully open.
In that case Gary T. was correct in post # 18 and now that I’ve though more about it I think that he was. When the OP is driving along at some speed on the freeway his throttle plate is closed to the extent that the engine can’t get its full charge of air. If you open the throttle you can get more air, and thus more fuel even at the same rpm.
As to your question I’m really not sure which is more economical. A general rule is that the higher the gear the better the fuel economy (with a carburetor) so dropping down one gear to allow less throttle at a higher rpm should result in more fuel being used. The question of the efficiency of the engine at various power outputs, throttle settings and rpm come in and who knows the effects of that?
I’m sorry for my faulty analysis and promise to improve in the future.
I never intend to make a mistake again.
Aw man, somebody give me a definitive answer here (no offence DS, you don’t seem too sure).
I’m clocking up some serious mileage at the moment and i’m also totally skint, so i really want to know whats the best thing to do.
The roads i drive on mean i’m often in the situation described, sometimes i can stay in top gear but i have to floor the gas to make it up the hill. After thinking about this recently i stopped doing that and started changing down a gear thinking that was better because i could ease off the gas.
Apologies for the slight hi-jack OP but it seems kind on topic.
If you have a standard transmission the rule of thumb is to get in the highest gear you can as soon as you can and stay there as long as you can without lugging the engine. What is lugging the engine? Well, you have depend on your feel for how the engine is doing. If you have got the throttle wide open and aren’t accelerating you should probably drop down a gear.
The main thing is to maintain as constant a speed as you can. Don’t jamb the accelerator down. Be light on it and make changes smoothly. Pretend like you have an egg that you don’t want to break under the accelerator.
This will require you to not pass slower traffic unless you have a lot of room, drive 60 or 65 mph on the highway and stuff like that.
And I am quite sure now, that Gary T’s answer in post # 18 was correct.
I imagine you meant to say “use more throttle” (i.e., press the pedal down farther), as otherwise your speed would reduce dramatically. Lower gear at a fixed engine speed means lower vehicle speed; lower engine speed in a fixed gear means lower vehicle speed; lower gear AND lower engine speed means MUCH LOWER vehicle speed.
No. More throttle = more fuel used = more oomph = (usually) higher revs. The OP question was about the occasional situation where the load is such that the oomph hits a wall, so to speak, and can’t generate more revs.
But as far as fuel consumption, David Simmons is correct - generally the higher the gear, the less throttle is needed to maintain a given (vehicle) speed, and the less fuel is used to accomplish that.
You don’t downshift to save fuel. You downshift to give the engine enough mechanical advantage over the load (vehicle weight, air resistance, speed, incline, etc.) so that it can maintain or increase speed as desired. This uses more fuel, but if the car can’t climb the hill at the desired speed in a given gear, a lower gear with higher revs and the attendant increased fuel consumption is the only alternative to losing speed.
The best fuel mileage over a given distance at a given AVERAGE speed is achieved by constant throttle. Sometimes this isn’t possible if the course is too hilly. The second best technique is to maintain constant speed with smooth, gentle manipulation of the throttle, as mentioned above. This is usually possible in normal driving to a greater or lesser degree, depending on conditions (other traffic, stop lights, etc.).
So if after flooring it in 5th gear at 30mph, additional air and fuel is being consumed with no increase in RPM, where does the extra fuel go? Is it somehow burned with no increase in car speed? Where does the energy from the extra fuel being burned go?
If I understand the thread then;
I suspect heat loss.
You’d be burning more gas (slightly more) and not removing the heat from the engine as efficiently as if you were cruising along at normal speed with a normal load on the car. Just as pulling a boat will cause some overheating of the engine.
But, if the engine can’t accellerate because of the load, then it can’t also suck more air into the manifold, thus, less air sucked in = less gas used (compared to if the load were off the engine). So this is a circular type of argument; If the engine can’t accellerate then it can’t really suck in more air (at WOT) it will use a little more gas (which is converted mostly to heat on loaded conditions) and slowly the engine will increase speed - which sucks in more air - which sucks in more gas, etc., until the torque catches up to the gearing and off you go.
No, it is totally relevant. At constant rpm, the engine is taking in a constant VOLUMETRIC flow. But because the manifold pressure changes over a large range the density of that air varys over the same large range. This means that the MASS flow rate is changing. This means that from a mass perspective, the throttle valve certainly does alter the flow rate.
I think the answer would also be affected (maybe greatly) by the parameters of the computer programming; how does it respond to increased airflow without a corresponding increase of RPMs and/or speed?
If the car really is burning more gas I’m guessing some of the energy might go into to drivetrain stress and vibration.
Gary T - No i did mean *less * throttle. When going up a slight hill at moderate speeds (i.e. low end of top gear range) in fifth gear i have two choices, either downshift which increases revs but doesn’t need much throttle as clearly the engine doesn’t need to work that hard to climb a slight hill in fourth. Alternatively i can more often than not stay in fifth but i may need to pretty much floor the throttle to keep the speed up and stop the engine from eventually stalling. You know that standard chug chug youre in too high a gear feeling.
I’m still not clear on the answer here as on one hand, i understand that higher gears are usually more economical due to greater wheel revs for given engine revs (you know what i mean), but on the other hand i’m told that the throttle dictates how much fuel is used so in my scenario, lower gear and higher revs would be more efficient.
To complicate matters more there seems to be the idea being talked about here that if the engine cant accelerate (as per the OP and my scenario) there wont be more fuel used regardless of the amount of throttle used. So flooring the throttle in my scenario doesn’t increase fuel use?
Pilots play this game all the time. We also have move levers, buttons and gages to see what is happening.
It all depends on the vehicle, the engine and what you are trying to do.
First, lets take out the stuff that adds needless complication for the question, supper chargers, blowers, turbo chargers, direct fuel injection ( like a diesel, straight into the combustion chamber ) and we will stay with carburetors and what most call fuel injection which sprays the fuel in just upstream form the intake valve.
We can see oil temp, head temp, exhaust gas temp, exact ground speed ( GPS ) we have very actuate fuel flow meters, manifold pressure gages and tachometers. We also have a constant speed transmissions ( constant speed propellers ) and we can control the mixture.
Now we can set it up where we take out as many of the variables as possible and see what is left.
We are looking for best fuel economy. ( slowest burn rate ) which will give us the longest time aloft.
We also are looking for the best miles per gallon.
we also have the restriction that we must remain in the air so that puts reasonable restrictions on practicality.
Now we go to an altitude where with a wide open throttle, we only can have say 24 inches of manifold pressure. ( throttle is out of the equation ) we set the RPM to say 2400. Now this is a good place for engine life and reliability for the most part on small aircraft piston engines. So, since we are straight and level and there is no wind and we are trimmed perfectly, we have taken the load ( wind resistance and grade ) out of the equation.
So now we have set the engine for the best known conditions of temp mixture and are getting a known burn with a known speed.
Now we will change just one thing, the load. We will use the propeller to slow the engine to 2200 RPM, ( increase the load ) and again reset the mixture to the best possible place. The fuel burn rate will be less. But we will be going slower, We can do this until we can just barely fly and we will get less and less fuel burned so our time aloft will be maximized.
*If we go the other way and increase RPM and reset to ideal conditions, we will be burning more fuel. There is also a point in this direction that the increase flue burn rate will not give enough increase in speed to improve the MPG.
This sweet spot is usually a very narrow range and within the confines of practical driving, you will gain more by constant throttle position than in increase in RPM.*
If we continue to reduce RPM, there is a price. There will be a point that we will be slowed down enough that we will actually cover less miles. There is a difference between best range and best duration. For each plane / engine combination, it is different but it is easy to see. There is always a trade off on what can realistically be done, you should not drive at about 40 MPH say on the interstate. You would need two lunches at least. For miles \ gallon, fixed throttle position is best within the restrictions of engine health, safety concerns, MPG etc. ( 25 in a 75 is not safe )
The computer controls in the newer cars do a great job with the bad situations that cars normally operate under.
For the real world, close attention to fuel burn over the same road day in and day out will show you what you can actually do that will get you the best MPG.
If all things are at their best efficiency, the only way to go faster or pull a heavier load is to use more energy. ( fuel )
Remember, a lot of theory is absolutely useless in practical application.
We also have totally left out the concern of the value of your time or the need to move in a fixed time.
*:: The only time you have too much fuel is when you are on fire. ::
:: If you want to go faster, you spend more money. :: *
Your missing the point. Picture this. You are in top gear at 60 mph on level ground. You hit a hill, and do not gear down, you just open the throttle more and more as the hill gets steeper and steeper. When you get to the point where the car is just staying even with the throttle floored, what would happen if you returned the throttle in the previous cruise position? Simple you would slow down. So by keeping the throttle wide open you are doing a bunch of work, and you are burning a bunch more energy due to the lower mechanical advantage of higher gear.
Bottom line, any time you add air you add fuel. On a gasoline engine the accelerator pedal is an air valve.
In Racing this is known as:
:: Speed is a function of cubic money, how fast can you afford to go?::
I find this contradictory. If you downshift, you get higher revs but you must increase throttle to maintain those revs. I don’t see how you can proceed up that hill in 4th with less throttle than you used in 5th.
This is different from your previous description which said “…needing to open half throttle to maintain speed…” If you can maintain your speed at half throttle in 5th, stay in 5th. Maintaining that same speed in 4th will require more throttle and use more fuel.
Now, if you’re talking about a situation where you can’t maintain your speed in 5th, even with the pedal floored, then it’s time to downshift.
When you sense that, which could be consistent with not being able to maintain your speed in 5th, it’s time to downshift. However, this is not in the OP description. He wasn’t talking about losing speed, rather about not increasing speed.
There does seem to be some disagreement about that. Rick and I have stated that in that situation there would be more gas used at full throttle than at part throttle. I’m at the limits of my knowledge on the issue, so I can’t offer a compelling argument to back up my contention.
Unless someone more knowledgeable presents evidence to the contrary, I maintain that more throttle always means more fuel use.
Torque (which is the quantitative measure that tells you whether and how fast you can get up the hill) is not a linear relationship with engine speed, or directly proportional to brake horsepower. Look at this graph. While horsepower is mostly linear between idle and 5k rpm, torque goes up a leveling slope, more or less flattens out between 3500-4500 rpm, and then takes a precipitous dive at 5k. So you’d like to keep this engine in the 3.5-4.5k torque band at a gear ratio that gives you sufficient output torque to overcome gravity and rolling resistance at the speed you want to maintain. Lower rpms at a higher gear ratio aren’t going to give the same output torque, so you end up driving it higher in the powerband (and feeding more fuel to keep it there) to achieve the same torque.
In short, engine RPM is not directly correlated to fuel consumption under an arbitrary load. Measurement of horsepower and torque under different standards (SAE, DIN, ISO, ECE) will differ somewhat even for the same engine, and the recent change to a new SAE test procedure (SAE J2723) made some changes to brake horsepower output for the same engine used in different model years of a car.
Stranger