Engine thermal output

So help me out here mathmaniac engineer types.  A buddy of mine has a 14,000 gallon pool, lives in the country, and has extra cars.

So how much heat would you be able to get out of let's say an SR20 motor?  What would be the ideal RPM?   How long to raise 14,000 gallons per degree?

And go! 

Not much load with the car sitting there.  How much gas does the motor burn at idle?  Take that and maybe 60% (30% if the car was doing something) of the energy capacity of the gasoline and that would be how much energy is available to heat the pool.

Dr. Hess (Forum Supporter) said:

Not much load with the car sitting there.  How much gas does the motor burn at idle?  Take that and maybe 60% (30% if the car was doing something) of the energy capacity of the gasoline and that would be how much energy is available to heat the pool.

Chuck an air-to-air intercooler into the pool and run the tail pipe into it. Now you can reclaim the heat from the exhaust too!

Assuming you are building a dedicated device you could probably take Dr. Hess's assumption up to maybe 80% if you ran the exhaust submerged in water so you extract that heat also.  

Also drive a pump off the motor that just circulates the water in the most inefficient manner possible with very restrictive discharge orifice.  That way you generate lots of heat through pumping losses.  

So lets say you made something that could burn 2 gallons an hour (which would probably require a lot of work to be done by the engine).  

That's 228,000 Btu's/hr.

One BTU is the amount of heat energy required to raise one pound of water by 1ºF, each gallon of water is ~8.33 lbs.  So your buddy has 116,000 lbs of water.  So assuming 100% of the energy went into the water he would get 1.96 deg/hr.  He won't get 100% efficiency so lets assume 80%, so he would get ~1.6 deg per hour.   

I'm not sure what the heat loss through the sides of the pool and the free surface would be but it may be quite high at lower temperatures.    That said 2 gallons of gas is currently cheaper then 67 KwH of electricity so he would be money ahead compared to submerging grill starters.  

I'd think a gas burner could be developed that would more silently burn fuel/waste oil to generate more like 400k BTU's and really provide some temperature rise.  

Pulling all of this from my butt as its been a while since I have done thermo stuff

How much HP do those make?  270ish?  So around 200kW?

Otto cycles have about a 55-65% efficiency rating if everything was ideal.  You could probably boost that up a minimum of 5% by doing some sort of reheat cycle with the exhaust.  So let's say real world probably 50%.

No clue how much water pumps will move, so assume you have a pump pushing one gallon per hour for ease of calculations.

Go from 50F to 100F?

(100kW * 3413)/(8.25*50F rise in temp) = 828 gallons per hour.

That's if all the HP is being converted into heat though, which is not going to happen as that is mechanical energy.

nocones said:

I'd think a gas burner could be developed that would more silently burn fuel/waste oil to generate more like 400k BTU's and really provide some temperature rise.  

De-icer trucks for airports use diesel, jet fuel, or gasoline burners to quickly boost temps of the de-icing fluid.  Usually have two or three burners per truck and each will run something like 1,000,000 BTU/hr.  They take an ethylene glycol and water mix from an ambient temp up to about 185F before being sprayed on the planes.

Always thought they were pretty cool.  I always wanted to work on them, but I was always stationed where it was warm.

This is like maximum redneck with a healthy dose of nerd in the mix. Another example of why I come here. 

I think building a direct gas fired thing would be far better.  Using a car, you would be drawing fully cold water into the engine, which would either slam the thermostat shut immediately, or with it removed, wouldn't get the output water very warm.  Add the exhaust stream, and things would get better.  Place the car on a proper chassis dyno and run it at it's torque peak constantly, stuff might start happening.  Insulate the pool walls and cover the top, and now we are talking.

 Some sort of direct fired gas burner that runs all the heat directly into a submerged heat exchanger  and lose almost none of the heat would be best by far.

Outboards use a thermostat of around 140F, wonder if it would be more realistic to use one of those as a base for this.  Then you'd get a jacuzzi effect as well.

nocones said:

Assuming you are building a dedicated device you could probably take Dr. Hess's assumption up to maybe 80% if you ran the exhaust submerged in water so you extract that heat also.  

Also drive a pump off the motor that just circulates the water in the most inefficient manner possible with very restrictive discharge orifice.  That way you generate lots of heat through pumping losses. 

This is the important part, by using heat from the exhaust and radiator you get almost all of the waste heat, but by doing this you can get all of the output energy as well, which is somewhere in the ballpark of 33% of the energy from the fuel. A much more expensive but proper way to do it would be to hook the motor up to a generator powering heat pumps and resistive coils, but there's nothing proper about this idea

nocones said:

Also drive a pump off the motor that just circulates the water in the most inefficient manner possible with very restrictive discharge orifice.  That way you generate lots of heat through pumping losses.  

 

My thought was drive a slew of junkyard alternators each hooked to an electric heating element.  You can toggle them on and off individually to tune the load.

Edit:  Hmm a stack of torque converters might do also.

Or spin some driveshafts with dragging brakes... but this energy would be difficult to harvest.

Fastest heating- 

A chasssis dyno where the power ouput i put directly into heat- big bank of resistors.

A water-water heat exchanger for the coolant.

A massive air-water heat exchanager for the exhaust.  

And run at peak power- so you are using most of the fuel as possible.

I would imagine you would get a vast majority of the heat generation that way- absorbing the mechanical energy, and getting as much of the heat losses as you can.

Othewise, use a flame to direcly heat the water.

I think there's also some heat to me reclaimed from the outside of the engine block. The car should be parked in the pool ideally, with as much of the engine submerged as possible.

slefain said:

I think there's also some heat to me reclaimed from the outside of the engine block. The car should be parked in the pool ideally, with as much of the engine submerged as possible.

That's where my mind went, too

slefain said:

I think there's also some heat to me reclaimed from the outside of the engine block. The car should be parked in the pool ideally, with as much of the engine submerged as possible.

If the pool water is flowing through at <100F, there isn't going to be much heat shed through the exterior surfaces of the engine.  Just insulate it and the plumbing the best you can.

Many many years ago I saw a system at a home show for co-gen heat and power.  Basically you installed a 3 cylinder diesel engine that ran a semi-significant generator.  During the winter it would run on demand and the coolant running through the engines water jacket was what heated the home through radiant heat floors.  The generator pushed more electricity out than the house used so it backfed the grid and earned the homeowner a credit from the power company.

It claimed to pay for itself in 4-8 years (depending how far north you lived).  I always thought it was a nicely elegant way to do things.  Multiple birds one stone sort of thing.

ProDarwin said:

slefain said:

I think there's also some heat to me reclaimed from the outside of the engine block. The car should be parked in the pool ideally, with as much of the engine submerged as possible.

If the pool water is flowing through at <100F, there isn't going to be much heat shed through the exterior surfaces of the engine.  Just insulate it and the plumbing the best you can.

Seconded.  Heat won't build up on the outside until you get the entire system up to temp.  Unless we're back to having a thermostat in the system, then its going to heat up and slam shut repeatedly

Mr_Asa said:

ProDarwin said:

slefain said:

I think there's also some heat to me reclaimed from the outside of the engine block. The car should be parked in the pool ideally, with as much of the engine submerged as possible.

If the pool water is flowing through at <100F, there isn't going to be much heat shed through the exterior surfaces of the engine.  Just insulate it and the plumbing the best you can.

Seconded.  Heat won't build up on the outside until you get the entire system up to temp.  Unless we're back to having a thermostat in the system, then its going to heat up and slam shut repeatedly

Excellent points.

Now, I think we are ignoring the potential energy stored in the interior. If a copper coil were run through the interior, you could light the seats on fire for an extra BTU boost.

I've never done the math but I can tell you that you can get 1,000 gallons of water to boil in about 15 minutes holding a 300 HP motor at a steady RPM under full throttle on an water brake engine dyno.