• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

A basic but confusing engine question.

SteveSS

Well-Known Member
Local time
6:48 PM
Joined
May 28, 2013
Messages
5,042
Reaction score
7,755
Location
Colorado Springs
What makes an engine accelerate?
I understand stepping on the throttle gives more fuel/air to an engine but that seems like a secondary function. Meaning the engine is asking for more fuel so the fuel system gives it what it is asking for. I know our old tractor sped up by a lever advancing the spark. You certainly can't speed up an engine just by pouring gas down the carb. It's the chicken or the egg paradox. So what makes it accelerate in the first place? Everything seems secondary, fuel/air, spark, etc.

The internet says fuel/air but is the engine just rearing to go waiting for the throttle to open?
 
Last edited:
What makes an engine accelerate?
I understand stepping on the throttle gives more fuel/air to an engine but that seems like a secondary function. Meaning the engine is asking for more fuel so the fuel system gives it what it is asking for. I know our old tractor sped up by s lever advancing the spark. You certainly can't speed up an engine just by pouring gas down the carb. It's the chicken or the egg paradox. So what makes it accelerate in the first place? Everything seems secondary, fuel/air, spark, etc.
You could probably pour gas down the carb and make it accelerate as long as you open the throttle to ad air. The old tractors didn't have mechanical or vacuum advance. They had manually adjusted advance. Just like Model T's.
 
Basically when you open the throttle blades on an ignition fired engine you are allowing the cylinders to injest more of the correct air/fuel mixture. Therefore allowing each combustion event to make more pressure and push harder and or faster.
A diesel takes a full breath of air every intake stroke , it modorates speed and power by varying the amount of fuel it injects. A diesel can run away on its own engine oil if the turbo seals are bad enough, or if the rack gets stuck open.
 
What makes an engine accelerate?
I understand stepping on the throttle gives more fuel/air to an engine but that seems like a secondary function. Meaning the engine is asking for more fuel so the fuel system gives it what it is asking for. I know our old tractor sped up by a lever advancing the spark. You certainly can't speed up an engine just by pouring gas down the carb. It's the chicken or the egg paradox. So what makes it accelerate in the first place? Everything seems secondary, fuel/air, spark, etc.

The internet says fuel/air but is the engine just rearing to go waiting for the throttle to open?
The combustion is a reaction.
It won't start that reaction by itself, that is why you have to start it. Used to do it by hand, then we got fancy and used a starter with a battery. Some old tractors had a pony motor. One in particular was started with a shotgun shell.
ICE is a meager attempt at a perpetual motion machine. It isn't, obviously, which is why it has to consume fuel. Fuel/air is the energy injected to keep the motion going.
Efficiency of the engine determines fuel needs.
If we ask it to work, it requires more fuel to keep the reaction going.
More fuel means more reaction. If there is no work to absorb that extra reaction, the engine spins faster because you have exceeded the energy input needed to perpetuate the motion. Starve it of fuel, the motion comes to a halt.

That is an overly simplistic description, but covers the bases. Most updates to ICE over the last 80 years have been to increase efficiency. Either more power for same fuel, or less fuel for same power, with the "work" being done as part of that factor. In the last 20 years, most updates were to allow same or more power out of crappier energy input(gas is NOT what it used to be) or to reduce byproduct of the reaction.
 
What makes the engine accelerate? The short answer is, opening the throttle as the accelerator pump squirts additional fuel so it doesn't stumble or die. LOL!
 
What makes an engine accelerate?
I understand stepping on the throttle gives more fuel/air to an engine but that seems like a secondary function. Meaning the engine is asking for more fuel so the fuel system gives it what it is asking for. I know our old tractor sped up by a lever advancing the spark. You certainly can't speed up an engine just by pouring gas down the carb. It's the chicken or the egg paradox. So what makes it accelerate in the first place? Everything seems secondary, fuel/air, spark, etc.

The internet says fuel/air but is the engine just rearing to go waiting for the throttle to open?
Your running engine is a balancing act. It takes a certain amount of power to spin the rotating assembly, plus accessories such as the water pump, alternator, oil pump and (if equipped) input shaft on the torque converter. Let's say in this case, 20 hp. At idle speeds with the throttle closed, just enough fuel/air mix is admitted under very low pressure (closed throttle creates vacuum) to provide that amount of power. Each molecule of burning fuel provides heat which is mostly wasted, but the useful part of it creates expansion in the cylinder to push down the piston. There isn't much push with a closed throttle.

If the engine surges or speeds up a bit, and the throttle plate hasn't moved, then the vacuum increases which means even less fuel/air is admitted and the engine slows down again. Idle speed dropping similarly does the reverse - the vacuum decreases (absolute manifold pressure increases) so the engine speeds up again to find the sweet spot.

When you depress the throttle pedal and allow more fuel/air mix in, then the engine produces more power than it needs for idle speed and the rpm's increase as a result. They will continue to increase until the power produced again matches the power required to rotate at that higher speed. Of course, if the car is driving, then the power required, in addition to that described above for an idle situation, will include air and tire resistance and drive-train friction losses.
 
If you've ever had a runaway diesel you'd understand. Feed it and it will roar...
Diesels are a little different.. They don't limit the airflow through the engine so adding just fuel makes the engine accelerate... A lot of run aways are actually due to failed seals on the Turbo or Supercharger... Diesel will burn lube oil just as well as it'll burn fuel oil.... If that happens the only way to stop it is to block the airflow... Or wait for the rods to ventilate the block...
 
Pumping losses. It takes energy for the engine to pump air and rotate the engine assembly. Take away the energy it slows down.
 
What makes an engine accelerate?


The internet says fuel/air but is the engine just rearing to go waiting for the throttle to open?

Essentially yes. It's idling at whatever rpm due to airflow restriction.

Yes timing/air fuel ratio will influence idle speed/vacuum but idle speed is largely adjusted by a screw that is changing throttle blade closed position and therefore airflow.

So at idle you have the weight of the atmosphere at 14.7psi (at sea level) available -above- the throttle blades to fill the induction tract/cylinders but the throttle blades are restricting that airflow which results in a vacuum -below- the throttle blades.

Opening the throttle is reducing/removing that restriction and ideally bringing the intake tract as close to atmospheric pressure as possible. (less/slightly less than atmospheric at WOT is common and a result of inefficiencies/restrictions or pressure drop across components)

The increased airflow must be accompanied by the correct amount of fuel to accelerate efficiently (maintaining correct air/fuel ratio for efficient combustion).

As well as having correct timing advance vs/rpm, simplistically.
 
More air and fuel and away it goes. Take the air away and it slows or stops, how you stop a runaway diesel. Without air pressure drop a carburetor fuel will not flow.
 
This might be related but we can't use more than a 600cfm carburetor at my elevation of 7500. So is there not enough air pressure or not enough O2 molecules?
 
This might be related but we can't use more than a 600cfm carburetor at my elevation of 7500. So is there not enough air pressure or not enough O2 molecules?
Sure you can... Just need more cubic inches... Or more foot control...
 
My carbureted car that runs the best is the 71 Challenger 408 stroker from Blueprint Engines with a 600cfm. The 97 Camaro fuel-injected 5.7 runs fine here but it has soooo much more HP at lower elevations. Back in the day when I lived in KS and OK it was necessary to have the carbs worked on when we visited CO. I've never had the Challenger at lower elevations. The shop that tuned the 408 said he wished he could order different cams from Blueprint to better match our thin air.
 
Last edited:
My carbureted car that runs the best is the 71 Challenger 408 stroker from Blueprint Engines with a 600cfm. The 97 Camaro fuel-injected 5.7 runs fine here but it has soooo much more HP at lower elevations. Back in the day when I lived in KS and OK it was necessary to have the carbs worked on when we visited CO. I've never had the Challenger at lower elevations. The shop that tuned the 408 said he wished he could order different cams from Blueprint to better match our thin air.
This is why they do forced induction for Pikes Peak
It removes the "thin air" issue.
 
Diesels are a little different.. They don't limit the airflow through the engine so adding just fuel makes the engine accelerate... A lot of run aways are actually due to failed seals on the Turbo or Supercharger... Diesel will burn lube oil just as well as it'll burn fuel oil.... If that happens the only way to stop it is to block the airflow... Or wait for the rods to ventilate the block...
Oh yeah, a runaway Detroit Diesel with a stuck rack; just starts upping the RPM’s. Used to use the driver’s logbook to shut off air thru the supercharger. When those cubes start going above 5k, it starts getting hairy!
 
Oh yeah, a runaway Detroit Diesel with a stuck rack; just starts upping the RPM’s. Used to use the driver’s logbook to shut off air thru the supercharger. When those cubes start going above 5k, it starts getting hairy!
A detroit spinning 5K... I'd be running away as fast as I could... The Navy boats had tachs that suggested 2300 was max normal RPM's & anything above 2500 was considered "Battle" speed....
 
I think most every DD had an emergency air shutoff. I didn’t know they existed until I accidentally triggered it once. I thought I was in big trouble but the boss opened the hood and opened it back up. Away I went.
 
Auto Transport Service
Back
Top