How Alternators Rob Horsepower

Many people will be surprised to know that your electrical system affects the amount of horsepower your car can produce. If your vehicle uses a lot of high current draw electrical devices, you’re not making as much power as you could be due to the significant drag produced by your alternator.

An automotive engine runs many accessories that draw power from it. Some of these accessories are the water pump, the air conditioning compressor, the power steering pump, and what we’re talking about here today, the alternator. Each of these devices is connected by a belt to the crank shaft of the engine. The engine not only has to produce the power to move the car, but it also has to produce the power to run all of these additional systems. The engine has to turn all of these accessories at all times, not just when the car is moving. From the time you turn the key, the engine is working to run these systems.

Alternator light

This light usually means that your alternator is not charging.

What does the alternator do?

The alternator is the device that generates the electricity to run the vehicle and all of its electrical accessories. On older vehicles this used to be called a generator. When these were called generators, they made DC electricity and they were pretty inefficient. For about the last 50 years, we’ve used alternators that produce AC electricity that is then rectified to DC. This type of system is much more efficient and provides more usable electricity than generators did.

automotive charging

This is the basic charging circuit.

What does the alternator power?

Every electrical device in your vehicle relies on your alternator. When you plug your phone into the charger, it’s the alternator that is making the electricity for it. When you turn on your headlights at night, it’s your alternator that is powering them. When you install that 1000W sub-woofer in your trunk, the alternator will be called upon once again to produce the power to run it. Every device, even the spark plugs, relies on the alternator for electrical power.

Doesn’t the battery power all of that?

Many people seem to think that the electrical systems in their cars are powered by the battery. It’s easy to see why people would think this since your car won’t start when your battery is dead. The truth is, once your car is running, you don’t need a battery at all. A modern vehicle is perfectly happy to run all day without a battery in it at all once it is started. The only time your car battery is used is when the engine isn’t running. The biggest use of your car battery is to run your starter motor to get the engine running. Once the engine starts, it’s turning your alternator, and the alternator is producing the power for all of your systems.

If your car really was running off of the battery, your battery would be depleted pretty quickly. Many electrical systems in today’s cars won’t work properly on less than 10-12 volts. A typical automotive battery that isn’t being charged by an alternator will get below a usable level in a couple of hours.

alternator parts

The parts of an alternator.

How can you make more horsepower?

An alternator is an electrical generator. It generates electricity based on how much electricity your car needs. One part of the alternator circuit is called the voltage regulator. The voltage regulator monitors the voltage level of your electrical system and tells the alternator to make more or less electricity.

The alternator is turned by a pulley with a belt that attaches it to your crank shaft. If your electrical system needs more power, the voltage regulator tells the alternator to make more power. In order for the alternator to make more power, it has to make the pulley harder for the engine to turn. This is taking horsepower directly away from your engine crank and turning it into electrical energy.

Once the voltage regulator senses that the electrical system is back to where it should be, the alternator pulley becomes easier to spin because it has less electrical load placed on it. This leads to the alternator requiring less HP to turn.

The bottom line-

If you run a lot of high powered lights and audio equipment you are reducing your engine power. All of these devices require extra electricity that is created by the alternator. The alternator makes this electricity by using horsepower and torque from your crankshaft to spin itself and generate current. The more current your vehicle needs, the more horsepower you lose to the alternator. If you use almost no electricity, your alternator takes almost no power to spin. In some bad cases, 15-20% of your engines total power can be used to turn the alternator.

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