Does car engine maintain RPMs??

If an alternator requires more current ... it becomes harder to rotate.. agreed

If an electrical load is added to your car alternator... the alternator is harder to turn ... which makes it harder for the engine to turn ... when this happens how does the engine continue to spin at the same RPM even though it is harder to rotate the alternator??

Say you took a lawnmower engine and rigged it to spin an alternator ... would it just keep losing RPMs at the electrical load increased?

Thank you

If you have cruise control set, and you increase the electrical load, your gas mileage will drop slightly. It's just like going up a slight hill.

As the load on the alternator is increased it will in turn place a greater load on the engine. At idle for example the ECU (Engine Control Unit) will adjust the engine RPM accordingly to maintain idle speed generally within a hundred rpm or so. However, since the engine is now doing more work there will be an increase in fuel consumption to maintain the same idle speed. That is also true if I am at idle on a hot day and turn on the air conditioning. The engine RPM will drop slightly and recover as the ECU compensates.

Now a lawnmower engine is a different animal unless it also has a throttle control. I have an old generator in the shed. It delivers 240 / 120 VAC at 60 Hz. The 60 Hz. line frequency is a direct function of te engine running at 3600 RPM. When I load the generator the RPM drops slightly and returns to 3600 RPM. The big difference between no load and load is to maintain the same speed of 3600 RPM the engine consumes more fuel.

If either the automobile engine or the generator engine did not have a form of throttle control to maintain RPM I would eventually stall the engine. That is unless I manually tried to maintain engine speed.

Bottom line is the more energy you make / use the more fuel you use at any given RPM. Cost more to drive up the hill than coast down the hill at a given speed.

Ron

yes if the electric current suck that hard but depends on the horse power of the engine

jareeb said:

yes if the electric current suck that hard but depends on the horse power of the engine

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It depends very little on the horsepower of the engine. The engine will produce additional power equal to that demanded. The additional power will be provided by burning additional fuel.

A lawn mower engine has a governor (similar to the cruise control on a car), as do most small engines, which tries to maintain the rpm as the load varies. One of the simplest is just a spring loaded vane connected to the carburetor, that is controlled by the air flow off the cooling fan. As the motor slows, the reduced air flow allows the spring to move the vane closer to the fan which also opens the carburetor. The speed setting is controlled by the throttle which adjusts the tension on the spring.

The other thing to consider, most automotive alternators are rated to produce ~50A at 14V at a couple of thousand RPM, which is 700W. One Horsepower is 746W, so allowing for some belt friction, the car engine will see an increased load of ~1HP if the alternator is maxed out. At idle RPM, the alternator is delivering only a few A, meaning less than 0.1HP, which the engine will barely notice... A 5HP lawn mower engine will notice a 1HP load, a 200HP car engine will not.

MikeMl said:

One Horsepower is 746W

Click to expand...

That's true with SAE hp. A DIN hp is 736W. (subtracting engine requirements (valve drive, cooling fan) to run the engine with no external load.)

SAE hp are being calculated from displacement and stroke. DIN hp are being measured at the drive shaft. (which is more realistic and honest?.)

For comparison: The Tyne MK22 aircraft engine develops 18,000hp at the turbine, of which are 5,000hp usable for the propeller. The rest goes for ignition, cooling, compressor power (18 stages) and mechanical losses.

Kind regards

Boncuk