Recapturing Waste Heat from Prius engine?

What you see is an exhaust heat, recovery system located downstream of the catalytic converters. When the engine coolant temperature is under 40C, engine coolant is heated by the exhaust to warm-up the engine.

Although it is attractive to speculate about a 'topping cycle,' something to tap the engine waste heat, they typically suffer from their own thermodynamic losses. So if you are after the 1/3d waste heat from the exhaust stream, you'd only be able with a 'perfect' heat engine get 1/3 * 1/3 ~ 1/9th the engine power. And you have to be careful that the downstream, 'topping cycle' does not reduce the efficiency of the original engine.

Worse, the thermodynamic efficiency of a heat engine is driven by the temperature ratios between the hottest and coldest parts (see Carnot cycle.) The two 'downstream', waste heat sources are at significantly lower temperatures than the combustion gas temperatures. This means you don't even get 1/3d of the energy from the 500-600C exhaust stream or the 85C engine coolant. But these low quality, heat sources could provide some uses.

It is not just a question of getting a little more energy from the 'topping cycle' but finding a place where it could significantly improve Prius performance. An ammonia absorption cycle could provide cooling in the summer to offload the existing cabin cooling. A turbo-alternator could provide additional 12V power to offload some of the electronics overhead. But one might also look at a solar panel to offset the parasitic electrical load from the keyfob receiver and memory maintenance current.

You might also consider how little energy the Prius normally uses:

A more productive approach would be to reduce Prius drag. Every 1 hp saved in drag reduction saves 2 hp of waste heat. So let's go over some of the 'low hanging fruit':

• larger diameter tires (NOT WHEELS!) on the front wheels - this reduces how much is lost due to transmission 'stirring' losses. In effect an over-drive gear.
• vanes on the engine compartment inlet - operated by engine coolant temperature, these reduce engine warm-up time, getting it into hybrid mode sooner, and reduce cooling drag by letting in only as much as is needed.
• wheel-well pants - this reduces the aerodynamic losses from air spilling out the wheel wells. Drive along side any car during a mild rain and watch the air spilling out of the both the rear and front wells.
• front and side skirts - this reduces the amount of air entrapped under the car, further reducing drag.
• side-to-side rear-view mirror and remove external mirrors (may be illegal in some states) - this can eliminate blind spots and reduce drag from the external mirrors.
• perfect, 4-wheel alignment - using shims and camber bolts, get the tires perfectly aligned.

Again, there is a 2-to-1 efficiency improvement by reducing drag. Topping cycles on the Prius are a lot of work and expense for little gain.

Bob Wilson

 

Very nice answer from Bob.
The first 5 minutes the heat losses are probably worse than OP showed (nice data in OP by the way).
What I try to focus on is quicker heat-up to improve the first 5-10 minutes of low MPG.
The idea there would be using waste heat to keep the car warm after shut down.

• Grill blocking redcues heat losses in winter
• Insulating engine compartment (moderator eFusco used to do that on his Prius)
• You could cover car with tarp when stopped (inconvenient)
• Possibly insulate exhaust better to keep catalyst temps up on shut down
• Engine block heater (cheating but I think Plug_ins like PiP could be equipped with block heater as standard equipment and automatically keep engine warm after the batt is charged and also do some heating of the passenger compartment)
• In regions where smog is not a concern, consider if modifying hybrid system warm-up cycle for better MPG on start-up is acceptable to EPA etc (non-CARB states must unite and request better MPG and 150k battery warranty!)

 

There is one experimental process that might use some of the exhaust heat, making water gas. The exhaust temperature AFTER the catalytic converter might be hot enough to sustain:

• C + H{2}O => CO + H{2} :: a fuel
• CO + H{2}O => CO{2} + H{2} :: more fuel with a temperature lowering CO{2}

However, it is very likely that the output of the catalytic converter is NOT hot enough for the +700C temperatures needed for the first reaction. Still, it is interesting as this could lead to a car that sort-of runs on water and carbon. In particular, the carbon black left over from refining. But I suspect the output temperatures of the catalytic converter are too low.

The NHW11 and NHW20 use fuel enrichment at high power settings to reduce the exhaust temperatures and keep from burning out the catalytic converter. Without taking actual measurements, we don't know if this approach would work. Then there is the plumbing and ducting problems at high temperatures. Worse, everything starts from normal temperatures, -20C to 35C and then has to reach operating temperatures, +700C, before the reactions can even hope to begin. Still, if a high quality, carbon source is used, the only by-products would be combustible gas, which could be cooled and fed to the intake air. Within limits, our Prius fuel injection system would simply tune by reducing the amount of fuel burned.

This is entirely theoretical and should not be considered as anything more than casual speculation.

Bob Wilson

 

Give it a try and let us know how it works out! Prius people love to read about those who experiment.

Bob Wilson

 

A video from Engineering Explained talks about a Merc. F1 Engine that claims 50% thermal efficiency... It seems to be a hybrid that uses a turbo-driven generator...