Compressor does not run ‘no matter what’

Dear Bob: Here’s a question for you relating to getting better gas mileage.

I know that, when you run your air conditioner, the compressor runs, which in turn decreases your gas mileage. But does it matter what coolness setting you have it on, since the compressor runs no matter what?

If I have the blower on low or the temperature on a higher setting, does it use less gas than if I run the blower full blast or use a lower temperature setting?

— D.L., Kansas City, Mo.

A: First off, it’s not true to say that the compressor “runs no matter what.” The compressor cycles on and off, and will be on more or less of the time depending on the cabin-temperature setting. This on-and-off cycling is quite evident in smaller vehicles with small engines, in which a drop in horsepower due to the AC compressor’s draw on the engine is noticeable each time the compressor turns on.

If the temperature is set on the low side, the compressor will run much more of the time than it would if it were set on the high side. In other words, the temperature setting inside the car is directly related to how much the compressor runs: The lower the setting, the longer the compressor will have to run to maintain the cool temperature. And the more the compressor runs, the more fuel it costs to run it.

This is not true of heat, though, since the heated air is drawn off the engine and circulated through the cabin, rather than being artificially heated. As you say, the blower fan is still operating, but at whatever speed its draw on the engine — and thus its impact on fuel economy — is negligible.

Dear Bob: Shame on you! Your response to R.W. of Garden City, Kan., regarding lowering the temperature of an overheating engine failed to bring the main point home.

There is no reason why a correctly functioning cooling system in any vehicle shouldn’t keep its engine at a normal operating temperature even under severe conditions. Running the vehicle’s heater with the AC off may, as you say, “make your passengers roast a bit” and may slightly lower the coolant temperature, but running an engine that’s overheated will no doubt lead to severe engine damage.

In other words, if it’s over temperature, there’s a problem. Your roasted passengers will be well done in the sun, as will you, waiting for the tow truck.

Why not steer your readers to a qualified mechanic who may hopefully be able to diagnose the engine’s cooling-system problems?

— J.R., Schenectady, N.Y.

A: Hello? That’s exactly the point of running the overheating vehicle with the heater on and the AC off — to get to a mechanic and get the problem fixed, rather than be stranded on the highway at the mercy of who knows what.

I never suggested that anyone should try to routinely run an overheated engine by using the heater. This is simply an emergency technique that can help a driver with an overheated engine reach the safety of a repair shop without running the engine in an overheated mode — which, as you say, can lead to serious damage.

You’re right, of course, in saying that a correctly functioning cooling system should be able to maintain an engine at a normal temperature even under severe conditions. But we’re not talking about a “correctly functioning cooling system,” are we? The original question concerned an engine that’s already overheating and, therefore, in need of immediate help.

And that, my friend, is the reason it pays to know this emergency method.

AMERICAN DRIVERS FIGHT BACK

Here’s a tip that will help you get better gas mileage while fighting terrorism and cutting our nation’s dependence on Mideastern oil:

Many drivers think that very slow acceleration from a dead stop is the most fuel-efficient way to get a vehicle moving. That’s not correct. The most fuel-efficient way to get a car moving from a dead stop is to use moderate to modestly brisk acceleration to somewhere between 35 mph and 45 mph.

Why that speed in particular? Because that’s the speed range at which most vehicles achieve their maximal fuel economy. The more rapidly a driver can move a car into this efficiency range, within reason, the sooner the vehicle will be achieving its top fuel economy.

Don’t get me wrong — I did say “within reason.” I’m not implying that you should slam the accelerator to the floor in order to get into the efficiency range as quickly as possible. Jackrabbit starts are prodigious fuel-wasters in their own right. After a pedal-to-the-metal acceleration, you’ll need a long time driving between 35 mph and 45 mph to make up for the fuel you’ve wasted by stomping on the accelerator to begin with.

So what we’re looking for here is a compromise, judicious use of the gas pedal to accelerate from a dead stop without going either too slow or too fast. Find that happy medium for your vehicle and get into the efficiency range, or as close to it as traffic permits, as soon as practical.