All Purpose Mechanic

What is the scope of the experiment? If you mean that the system consists of an engine and the generator in a large calorimeter with just two wires going to the outside (the load is external to the calorimeter) and the exhaust gasses are not allowed to leave the calorimeter, then the following happens:

Under no load the system will produce more heat for a unit of fuel than it will under load. The reason is that energy is neither created nor destroyed and when you add the external load to the system you are effectively drawing some energy out of the system to power the load (i.e., your ceramic heater). Under load the net heat produced in the calorimeter must be lower.

If you put the load inside the calorimeter and measure the heat produced for both no load and load the heat generated will be equal for the same quantity of fuel in both cases. This is a totally closed system.

Both examples must follow the law of conservation of energy.

Good challenge question if you were to refrase it as I describe it!

Anonymous Hero:

I agree 100%. Good analytical approach!

"Under load the net heat produced in the calorimeter must be lower." I would just add (I know what you meant but apparently at least one didn't): per unit of fuel used.

We know it will consume fuel at a higher rate under load.

Regards,

Greg

I agree.

In principal, I agree with Hero's excellent answer. However, his answer may be either more or less theoretical than you want... maybe.

If it is less theoretical, then we should also look into the energy in the fuel that the engine passes through, but does not burn, I think. This is really, really, nitpicky, I suppose, but your room will be full of unburned hydrocarbons, that until they condense, are not accounted for by the calorimeter...

But maybe you are looking for something far less theoretical, such as: at low load the engine will be less efficient, producing a different ratio of heat generated vs power supplied to the other room. In which case you need to look at the engine's bsfc curve. (As I'm writing this, I think maybe I need to reread your question...)

In any event, a neat question elegantly answered by hero.

Wow, here I am, talking to myself.

What I was getting to with bfsc, is that the question is a little ambiguous: "does same amount of heat" mean total heat (engine and heater combined) or just engine?

Except that the generator under load will heat up some adding to the enclosure's heat load.

I am not sure if you are saying that the calorimeter will be hotter under load than when no load is applied because of the generator heating up.

Energy into the system must be equal to the energy expelled by the system. That is simply one of the laws of thermodynamics.

Under no load, all of the energy is contained inside the calorimeter. When you add the load (externally) you are syphoning some of that energy out of the calorimeter, so the net heat (energy) inside the calorimeter will be less.

You can get the exact same effect if you simplify the experiment. Remove the Honda generator from the calorimeter and put in a steel cup and put all of the fuel into the cup. Ignite the fuel. The total heat produced inside the calorimeter will be the same as the no load generator.

Pop the lid on the calorimeter while the fuel is burning and you find that some of the heat escapes outside. The net heat inside will then be lower. That would be analogous to adding a load.

Basically, same heat of rejection per unit of fuel. More output per minute, more heat per minute. You need a bigger cooling system.

RichH

A slightly shorter version (I think) of Hero's answer:

Without load, all the heat from burning stays as heat in/around the engine. With load, some of the heat is converted to electrical energy, and appears at the load instead. So less of the available heat is dissipated in the engine itself.

Fyz