How does gas usage in a car work? What makes me use more or less gas?


So I know driving takes gas, and having my lights on costs battery power. But somehow driving charges the battery? Does that cost me extra gas? And what actually costs me gas also? Does standing still with the motor on cost me gas? If I have heating on max? If I have airco on max? What kind of driving costs more or less gas? I am so confused with this all. I dont feel like I am doing anything insane in my car, but still the total amount of km I can drive varies so much on a full gas tank. It goes from 600 to 400 and I have no idea why. I hope there is someone who can explain this to me

In: Engineering

It’s all about energy.

All of the things done by a car require energy. All of the energy in a car comes from the engine, with some of it pausing in the battery for a while before being used. It even costs some energy to keep the engine running.

All of that energy is roughly equivalent to gas. More energy means more gas.

Keeping the engine running uses some gas. Nothing crazy, but enough to be significant. Turning on your lights does as well. *Way* less energy, though – an insignificant amount.

While the heater does require energy, it actually taps off of the wasted energy of the engine and so does not use additional gas (but will still only work when the engine is running).

Accelerating the car takes a lot of energy. That’s a lot of gas. Keeping it moving also does this, but not near so much as accelerating.

The air conditioning also costs a fair amount of energy – it is the most fuel/energy-intensive subsystem of the car.

The engine use gas whenever it’s running, even if the car doesn’t move.

In a car, every system is powered either by the engine, or the battery, which is in turn recharged by the engine. Whenever something use power (be it to move the car or light the headlights) it will cause the engine to run “harder” to generate that power, or drain the battery, which will in turn require the engine to run “harder” to recharge it. Running the engine “harder” eats more gas.

One notable exception to this is the heating, which reuses the eat of the engine which is normally dumped outside.

> It goes from 600 to 400 and I have no idea why. I hope there is someone who can explain this to me

Gas consumption by km also depends on speed. Going faster consume more gas per km.

Other things that can add to your gas consumption are:

* AC. “traditional” AC is extraordinarily energy expensive and thus can increase your mileage by a shitton (You can actually feel this if you have a low power car, some vehicles are noticeably more sluggish with the AC on: I had to turn off the AC to be able to get to the top of a mountain once)
* Load. A loaded car is heavier and thus require more energy to move.
* Starting & stopping.
* Running the engine cold

Things like headlights do consume some amount energy, but it is very very small compared to the amount needed to get the car moving.

> Does standing still with the motor on cost me gas?

Yes, and this is the biggest factor. Whenever the motor is running, it’s continuously burning gas, that’s what is making it run: exploding the gas.

The variation is mostly caused by how much time you’re idling (standing still) vs driving. City driving uses more gas per km because you have to factor in all the time stopped at lights, driving slower in roundabouts etc. Highway driving uses less gas per km because you’re moving forward the whole time your engine is on and burning gas.

Also maybe in the winter you remote-start your car before going out to drive? The whole time it’s running in the driveway, the engine is burning gas and you’re going zero distance.

Other factors:

-Gas engines get the most power out of the gas in a pretty narrow range of engine spin speeds (rpm). So aggressive acceleration uses more gas to get to the same speed than speeding up gradually, because accelerating hard pushes the rpms above the most efficient level.

– The faster you go the more air resistance there is. There’s a certain speed with a balance between the best rpms for the engine efficiency meets the increasing air resistance of going any faster. The number depends on the vehicle. Driving faster than that uses more gas per distance than going slower.

– I’ll let others cover this better but yes heating/airco do (indirectly) use gas by draining the battery. Mythbusters did a study on this.

A lot of these answers are good, but they’re a little confusing, here’s a simpler explanation. Your car has an idle throttle, it’s the minimum throttle required to keep the engine spinning, I’ll explain the concept of throttle in a second. If the engine is spinning, it’s consuming gas. Tied into your engine are a series of belts/chains, these transfer rotational power to other parts of your car. Within every car, there’s a tiny electric generator called an “alternator”, this charges the car’s battery as it runs, this is run off of a belt attached to the engine output shaft.

I’m going to assume you’re speaking of more modern fuel-injected cars rather than carbureted. When you push on the gas peddle, it tells your Electronic Control Unit, ECU, to increases the rotation of your fuel pump, which provides more fuel to your fuel rail(s), and puts more fuel into the cylinders. This results in a larger explosion and more power and more RPMs for your engine. The harder you stand on it, the more fuel goes in.

Now, back to the electricals. Not everything runs directly off the battery, some are in-line with the alternator. Interesting thing about electrical generation, it’s all the same, magnets spinning inside a ring of other magnets. But how much electricity being demanded makes it harder to spin that inner ring, the inner ring of your alternator. This means that if you’re using a lot of electrical power in your car, the engine has to work harder to get you to the same speed, meaning you have to push the gas pedal harder, meaning you’re spending more gas.

However, depending on the car, this is nearly negligible. My old, old, old, ’92 Honda civic would noticeably chug with the AC on, but it produced maybe 80HP. The bigger the engine, the less noticeable this is.

If you want full on gas savings, accelerate slowly, stay in the highest gear that you reasonably can (Don’t pop it in 5th at 30km/h), keep the windows up and don’t run the AC. The heat runs off excess heat from the engine coolant, so you’re fine there. But it’s such a tiny saving, it’s better to be comfortable while driving.

Biggest factor (in real life) is the person in the driver’s seat. In a modern car, electrical consumption and even AC isn’t going to make a big difference in fuel consumption.

Limit top speed and hard acceleration. Learn to anticipate traffic and coast as much as possible to slow down rather than going full speed then hitting the brakes hard. This means not tailgating – keep a good distance between your car and the car in front. Although you cannot control traffic congestion, avoiding it will save a fair bit of consumption.

There is a huge difference driving at constant highway speeds and in-town traffic. So if the driving pattern changes a lot, expect fuel consumption to vary by a lot too. The load carried also plays a factor. Make sure the car is serviced regularly.

Lights are an insignificant power draw. All of your electrics are, unless you’re running a huge thumping stereo. The heater only costs you the insignificant power of running a fan since the heating part is just done by running your hot engine coolant through a small radiator. Air conditioning can sap a little bit off your mileage.

Most likely it’s the way you drive. You maybe press the pedal to accelerate faster some times, and others not? That can make a major difference in your mileage.

Top Gear once ran a Prius and BMW M3 on a race track. The Prius was to go as fast as it can around and around, and the BMW was to stay right behind it. But the Prius has a tiny engine so the driver had to drive hard, and the BMW had a big engine and barely had to touch the gas to keep up.

The result, the BMW actually got better mileage than the Prius, because the Prius was driven hard, and the BMW wasn’t. But if you drove both cars lightly, the Prius will easily get much better mileage.