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Reducing Losses To Heat

The first place your vehicle’s energy goes is the engine. For electric motors, there is some loss to heat and electrical resistance, as well as some loss due to conversion from DC to AC power in many models. For gasoline and diesel engines, though, this is where most of the energy is lost to heat.

Engineers have been working for over 100 years on reducing heat losses and finding ways to capture some of the lost heat energy, and have made a lot of progress, but the laws of physics cannot be broken. For this reason, the automotive industry is going to be mostly moving to electric vehicles in the next few years.

For now, the only way a gasoline or diesel car’s driver can reduce heat losses is to try to avoid burning fuel. You can do this by shutting the engine off when you can (standby on the flowchart), not pressing hard on the accelerator (this makes more heat), and making wise use of deceleration fuel cutoff (again, standby).

Starting a warm engine takes about 6 seconds worth of fuel in most cars. So, if you know you’re going to be sitting at idle for more than about 10 seconds, you can shut the engine off to save fuel. This also means losing air conditioning or heater, so this might not always be a good strategy.

Increasing your coasting time helps save even more gas, because most newer cars (1990s to present) have a feature called Deceleration Fuel Cutoff. When you are slowing down toward a stop or going down a steep hill in a lower gear, this feature stops sending fuel to your engine. During this time, the engine still sounds and feels like it’s running, because instead of fuel keeping it turning, your car’s momentum is turning it. It still pumps air in and out, but runs much cooler and uses no gas.

Hybrid gas-electric cars (including “mild hybrids” and start-stop systems) automatically shut off your gas engine when coasting to a stop, and automatically shut the engine off when you’re sitting still. Some cars with start-stop give you the option to disable the stop-start system, but don’t disable it! It’s saving you a lot of money.

Accessory (Parasitic) Losses

Once the engine is done wasting energy, engine accessories on the accessory belt take their cut. Common accessories include:

  • Alternator (charges the battery, powers headlights, radio, electronics)
  • Air Conditioning Compressor
  • Water Pump (helps take the heat out of the engine)
  • Mechanical Cooling Fan (if your car has one)
  • Power Steering Pump
  • Some emissions equipment in older cars (air pump, etc)

To reduce your losses, either

  • remove the accessory from the car (not always possible or easy) or
  • reduce your use of it (much easier for the average driver).

Removal or replacement of power-robbing parts is not within the scope of this lesson, so we will go over what parts the driver has control over and can use less to save gas.

Water pumps, mechanical cooling fans, power steering pumps, and emissions equipment are not within the control of the driver. Newer cars often do not have this equipment or have replaced the equipment with electric parts, so this might not even matter for your car. Consult a mechanic, your manual, a repair manual, or reliable internet sources to see what the situation is for your car, if you’re curious.

The two things you can control while driving is how much you use the alternator, and how much you use the air conditioning.

Reducing Alternator Losses

Alternators use more engine power the more they’re being used. So, to save power and gas, use less electricity.

The first thing to look out for are big power sucking devices. DC-AC inverters (gives your car a house-style plug), high-powered stereo systems, big off-road lights, and other aftermarket equipment are good examples.

A good rule of thumb: if the headlights get dimmer for a bit when you turn something on, or if you hear the engine struggle for a bit when you turn something on, the thing you turned on is burning up your gas. If you can do without that thing, don’t use it.

Another thing you can do to reduce the electric use is convert your car’s light bulbs (including the headlights) to LED. Many auto parts stores and internet retailers sell LED replacement bulbs. Replacing bulbs alone probably won’t give you a noticeable bump in gas mileage, but combined with many other small things, can make a difference. LED bulbs also last far longer than regular bulbs, so you’ll save money in the long run either way.

The Air Conditioner

The best thing you can do to save here is to not use the air conditioner. Now, before a crowd of angry, sweaty people strings me up, I’m not saying to never use your AC! If you need it, use it. But, make sure you actually need it when it’s on.

If the ambient temperature outside is comfortable, you can probably get by on just the “vent” setting, or by turning AC off and keeping the fans on. If it’s cold out, definitely don’t use the AC!

There’s also the equipment all cars come with: the “450 AC Unit” or “250 AC Unit”–the old joke goes (2 windows down, 50 miles per hour, or 4 windows down, 50 miles per hour). For around town in many conditions, simply rolling down the windows can make you comfortable without the AC off.

But, don’t roll the windows down at highway speeds. The extra aerodynamic drag caused by rolling the windows down will cost you more gas than the AC would.

When using the AC, save on gas by keeping the fans as low as you comfortably can (they use electricity, and thus gas).

Finally, keep your AC from having to work hard. Use windshield shades to keep the inside of your car cooler when parked or keep it in a garage/carport. That way, when you turn the car on, you don’t have to blast the AC to cool the car off. Also, use the recirculation setting on the AC to keep your AC unit from having to work as hard to keep you cool once you’re under way and it feels good inside.

Hybrids and Electrics

Accessories like heat and A/C draw power directly from the drive battery. Using them reduces range, so be sure to use them wisely.

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