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Accessory Load

The accessory loads that we are talking about are the items that are connected to the Fan Belt, Accessory Belt or Serpentine Belt. These items are the Alternator, Water Pump, Power Steering Pump, Air Conditioner Compressor (usually only on passenger vehicles) and Cooling Fan (Usually on older vehicles when the cooling fan is connected to the fan belt. Newer vehicles have electric fans). These items take power from the engine while they are running. Most of the items are always running, but the Air Conditioner Compressor only runs when you have the Air Conditioning on or the Defroster is running. Some older vehicles built in the late sixties to early eighties also had "Smog Pumps" which pumped air into the exhaust to further burn unburned hydrocarbons.


The Alternator generates electricity and works with the battery to run the electrical items of you vehicle. An alternator is going to be on almost all vehicles, including cars, trucks, race vehicles, motorcycles, and all other vehicles with engines. Alternators usually have medium Accessory Loads.  Note:  The Alternator load of computer controlled cars is significantly higher than older carburetored vehicles.

Water Pump

If you vehicle is liquid cooled then it will have a Water Pump. The easiest way to tell if you vehicle is liquid cooled is to look for a radiator. But some vehicles donít have radiators such as snowmobiles. They use heat exchangers usually mounted under the seat. Another way to tell if you vehicle is liquid cooled is to look at the engine itself. If it has cooling fins on the engine then it is probably not liquid cooled. Water Pumps can have significant Accessory load, especially at higher RPM.

Power Steering Pump

Power Steering Pumps are mostly used on Cars, Trucks and Boats. Smaller vehicles such as motorcycles, snowmobiles, go karts and ATVĎs donít need Power Steering Pumps because of their size and weight. But some of the larger ATVís are not being built with Power Steering but they are usually electric and therefore the Accessory Load is very minimal. Power Steering Pumps typically have a low Accessory Load unless you are steering the car.

Air Condition Compressor

Older Air Conditioning Compressors were a "power hog". They sucked up horsepower and significantly reduce gas mileage (while the air conditioning or defroster is running). Newer Air Conditioning Compressors are much more efficient.  The factories say you actually get worse gas mileage with the windows down (which increases aerodynamic drag) and the AC off than with the windows up and the AC running.  That is why most racing vehicles do not have air conditioning. The Accessory Load of a running Air Conditioning Compressor can be high.

Cooling Fan

On older cars and trucks the Cooling Fan was connected to the Fan Belt. This configuration was very inefficient. The Cooling Fan pulls air through the Radiator when the vehicle is not moving. But when the vehicle is moving, the need for a Cooling Fan is reduced. In the late 70ís the Clutched Fan came into play and reduced the drag by allowing the Fan to slip at higher engine RPM.  The Flex Fan has blades which bend at higher RPM, taking less "bite" out of the air a higher RPM.  Their drag is also reduced at higher RPM.  And now most vehicles use Electric Fans that only come on when needed, being the most efficient.

Programs the take Accessory Load into account

Accessory Load needs to be taken into account when implementing Engine Simulation and Performance Trends has 3 programs that will help you figure out Accessory Loads. Engine Analyzer v3.4, Engine Analyzer Plus v3.4 and Engine Analyzer Pro v3.9.