So I’m driving my car down the freeway, and my stickshift is in fifth gear (a miracle sometimes on the Dallas freeways). I’m watching my tachometer. I notice sometimes that, when I’m driving slow enough that I really should be in fourth gear, when I push the gas pedal to the floor the tachometer doesn’t move.
My question is this - since the engine isn’t speeding up, is my gas consumption rate increasing or staying about the same?
It is increasing.
The gas pedal pushes more gas into the engine…
If the car doesn’t speed up, that just means that the engine doesn’t have enough torque at the gear ratio to speed the car up, not that it isn’t trying.
So even if the tach doesn’t move, the engine is moving faster?
The way I understand it is this: With each cycle of the engine, one or more cylinders get a shot of gasoline, which is then compressed halfway through the cycle prior to ignition. This squirt of gasoline is a measured amount - too much and the gas won’t have enough O2 to burn efficiently, too little and there’s no serious bang. Thus, if you have 3000 revolutions per minute, then each cylinder gets (I know I’ll get this wrong) 1500 squirts of gasoline per minute, which comes out to the same amount of gasoline whether the throttle is open halfway or all the way.
If the gas pedal is pushed all the way down, but the tachometer says that I’m still at 3000 RPM, how is the engine getting more gas per minute? Or, more accurately, how badly broken is my understanding of internal combustion engines? 
In the end, the only way to speed the car up is by putting more gas in than is needed for the current RPM. But if the car can’t achieve a greater RPM regardless of how much gas you add, then simply it’s not going to change.
Er, so no. The engine isn’t going faster. It’s just squelching the extra gas.
Nope, you’re just pumping more gas per cylinder. Think of it this way; if you’re on a steep decline and using the engine to brake (thereby only giving it just as much fuel as the injection system delivers on idle) you’re burning substantially less fuel than at the same RPM going uphill.
Plus, when you’re flooding the injection system by pushing the gas in high gear, most of the fuel isn’t being burned. You’re just dumping it out the tailpipe. (Modern injection systems will limit fuel input as well as add additional air to get as close to complete combustion as possible, but still.)
Stranger
Aha! I think this highlights a flaw in my understanding of car engines. I had assumed that when gas is injected into the cylinder, it’s a fixed amount. Is this reasoning wrong, then?
No mechanic, me; but as I understand it it is the ratio of fuel to air that is more or less fixed. About a 15:1 air:fuel ratio is needed to run an engine. (I’m sure there are many, many variations. As I said, IANAM.) By adding more fuel and more air, you produce a bigger bang and more power.
Yes. There’s no other way to create more energy than to add more of the boom.
Nope. When you push on the accelerator, more fuel goes to the engine, regardless of speed (although the end result is usually that the engine speeds up if it is free to do so). The fuel injection system compensates by drawing more airflow into the intake manifold. (Direct injection engines work a little differently, by spraying fuel directly into the cylinder, but the result is the same; the more fuel you put in, the more air you need to pull in through the intake in order to maintain a stoichiometric combustion ratio. Older carburated engines had a strictly mechanical device to do this, and it was rather easy to “flood” the engine with too much fuel to be mixed and burned, stalling the engine at startup or low speeds. (Flooded engines, like worn ignition points and broken rotor caps, are now a thing of the automotive past, of interest only to restorers.)
Stranger
14.7:1 is considered a lean mixture, and in the power band most engines run something closer to 13:1. The thermodynamic effecienty of turbocharged/supercharged engines allows higher ratio/leaner mix with greater compression, yielding more power output per unit of fuel closer to the stochiometric ratio.
Stranger
Come on Stranger, us poor folks with old cars and trucks are not gone yet. Go to any automotive parts store and check, you don’t have to go to a ‘specialty house’ yet, to get points and distributor caps.
With older carborator systems you would use more fuel, with modern fuel injection systems I’m not so sure as the computer gets involved and computers have a mind of their own. The computer knows at the given loads and speed what is the max amount of fuel the engine can take, if you are already at that limit stepping down on the gas won’t do anything.
Sorry Rick, but you’re wrong. In the case Subway Prophet describes, having the throttle wide open reduces pumping loses and increases mileage.
The gas pedal opens the throttle plate, allowing more air to flow through the engine. The flow of fuel is regulated based upon the airflow, not the RPM, although on modern, computer-driven cars the RPM is taken into account by the fuel injection system, along with a host of other data.
When you push the accelerator down, a shot of extra fuel is pumped into the engine, momentarily enriching the mixture to provide the impetus for increased RPM, thus increased airflow, thus faster speed. If your gear ratio is such that you cannot overcome inertia to increase RPM, you waste this extra shot of fuel.
However, there is a maximum amount of fuel deliverable for any amount of airflow. If you are at, or near, this ratio, opening the throttle plate (and holding it steady) will not increase the fuel flow. In fact, you may decrease the fuel flow in a conventionally carbureted engine, because of a reduction in air pressure across the fuel ports. In this case, the power output and RPM may actually decrease.
Lots of misinformation here.
First off think of the engine as an air pump. The throttle controls how much air is allowed into the engine. The fuel injection computer calculates based on the amount of air being admitted into the engine how much fuel to inject.
So if you put more air into the engine, you will have to inject more fuel to maintain the mixture.
As has been mentioned, 14.7:1 by weight is the stoichiometric mixture for gasoline. This is not the best overall mixture for economy, or power. But is the best overall for emissions. The computer will add additional fuel when the throttle is opened above a set percentage to provide additional enrichment and prevent stumbling when accelerating hard. (For Gus, think accelerator pump in a carb)
Likewise when the throttle is almost closed, very little fuel is being injected since there is very little air being admitted into the engine. If you close the throttle completely the injection will cease until a preset lower limit, when injection will resume (say 1300-1800RPM)
So anytime you open the throttle you will inject more fuel. If you are in too high a gear, you will either not accelerate, or accelerate very slowly, but in the mean time you will be using more fuel.
Dog80 in the real world, the full throttle enrichment from the injection will wipe out any gain you would see from no pumping losses.
It may help to realize that “gas pedal” is a misnomer (unless it’s a diesel). The accelerator pedal directly conrols the amount of air allowed into the cylinders. The carburetion (administering of gasoline into the cylinders) is a response to that airflow. A carburetor achieves this through a simple mechanical process based on Bernoulli’s principle; a fuel injection system achieves it through an interplay of sensor, computer, and injector(s).
Usually, when you tromp on the accelerator, the additional air and the fuel that accompanies said air cause the engine to produce more power and thus increases its speed. In some cases, such as that mentioned in the OP, the engine is at a mechanical disadvantage that does not allow that response, but nevertheless the additional air and fuel is being consumed.
A rough analogy would be pushing harder on a lever where the fulcrum placement is such that even with all your strength you can’t budge it. You still consume energy trying. If you reposition the lever (e.g., shift into a lower gear) you can then make it move when you press harder.
Er, ah, um … with carburetor, when you step on the accelerator you also actuate an accelerator pump that gives an extra shot of fuel to the engine. However, in order to make use of this fuel the engine must speed up.
If the engine is running at some speed, say 1800 rpm, there is a dertain throttle opening and this results in some manifult pressure, or rather vacuum on an unsupercharged engine. Stomping on the accelerator opens the throttle and increases the manifold pressure, or reduces the vacuum so unless the engine speeds up less fuel us taken out of the nozzle in the venturi throat of the carburetor. Your engine will just sit and cough and jerk and do all sorts of bad things unless you shift down so as to allow th engine rpm to increase. The increase in rpm restores the correct manifold pressure and allows more air to flow through the carburetor increasing fuel comsumption.
I’m not that up on fuel injected system but I suspect the might work about the same. A computer controlled fuel injection system is a different breed of cat and others will have to take over from here.
The brief squirt from the accelerator pump isn’t really germane to the particular question here, but while we’re on the subject…
That extra fuel complements the sudden rush of air going to the cylinders, and is used along with said air. The reason for the accelerator pump (or its equivalent depending upon the particular design) is to get around the time lag for the main carburetion sytsem to respond to the increased airflow.
I don’t believe that’s right. In the situation described in the OP, whether with a carb or injection, there is no bucking. The carburetion system responds to airflow, not to manifold vacuum.