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Rotary Engine Returning to Mazda in 2019... as a hybrid

2K views 7 replies 3 participants last post by  michael95350 
#1 ·
There have been rumors lately that Mazda's R&D has been resuming research on the Wankel rotary engine. The latest rumor is that the infamous engine will make a return in 2019. However, it will not be the primary powerplant. Instead, it will be used as a hybrid engine to extend range of an electric vehicle it is planning to build. What is even more interesting is that Mazda claims they are NOT working with any other manufacturers on this technology.

Read all the details here
 
#2 ·
#4 ·
I guess I should read titles better, would make eating crow less likely. Anyhow, hard to see it come back in any form unless it can meet crazy emissions standards.
 
#5 ·
It's possible to meet emissions standards of virtually any stringency IF you can restrict the operating paradigm of a given engine to where its emissions are under best control. That's what a Hybrid design makes possible; the engine only runs at the RPM and load factor(s) where it operates "in the zone" that is clean enough, since it is turning a generator.

The problem with such a design is that it is inefficient since you must convert the power twice -- once to electricity and then back to rotational energy in the motor, and ALL conversions involve loss. Even the best PWM-driven PM motors lose ~10%+ of their input energy as heat.....
 
#6 ·
And therein is my issue with any hybrid system. For years, I've been one of those "crackpots" that has supported hydrogen fuel stacks as an energy source. Naturally, the biggest issue is (lack of) infrastructure. However, I think the last few years has seen some major possibilities come forth. There's even a hydrogen refuel station a stone's throw away from my commute route.
 
#7 · (Edited)
That's a crackpot fuel cycle when you get down to it.

The "best" (thermodynamic wise) path is this:

CH4 (natural gas) -> Reformer (2H2 + CO2 [released to atmosphere]) -> Compressor (big and expensive) -> Gaseous H2 tank -> Regulator -> Fuel Cell (2H2 + 2O2 -> 2H2O + electricity) -> Storage/Buffer -> Motor

The losses are fairly nasty. The reformer requires energy to operate. The compressor is nasty-expensive because hydrogen is a very small molecule and thus keeping bypass (leakage) to a reasonable level costs a great deal of money, the output must NOT contain any contaminants (they will poison the fuel cell) so the filtering (under extremely high pressure) is expensive and the tank is both expensive and has a recurring hydrotest requirement plus BIG safety issues (if you want to see what sort of damage a ruptured tank does look at the aftermath of a scuba tank explosion, then envision a vehicle collision that could damage the integrity of the tank!) Extremely high pressure is required to get reasonable amounts of hydrogen gas in a usable volume. Let's assume we're REAL GOOD and can do that with only a 10% penalty (that is, 10% of the fuel value is consumed by compressing it for the tank.) For cost comparison purposes there are consumer-level CNG compressors available for vehicles that run on it; they cost a few thousand dollars and have NONE of the contaminant sensitivity that a fuel cell does; in addition CH4 is a MUCH larger molecule and thus the tolerances to keep bypass reasonable are a fraction of those required for hydrogen. $10k is an extremely aggressive (but maybe possible) cost target for a "home sized" compressor that can fill a vehicle's tank in an hour or two. For larger "fuel station" style units with significant volume capacity you're talking about a LOT of money.

But the bad news doesn't end there. The reformer is only about 70% efficient. Fuel cells are ~50% efficient -- that is, half the energy in the hydrogen you put in goes to waste; only half is returned as electricity. Losses are multiplicative; 0.9 * 0.7 * 0.5. So at the output of the fuel cell we have 31.5% of the energy in the original natural gas available to us in electricity.

Note that a fuel cell generally has trouble meeting very rapid load changes, so it needs a buffering mechanism in the middle. You get hosed fairly badly there when its in use too -- in fact, very badly, as charge/discharge efficiency for batteries is in the ~80% range *in the bulk charge/discharge regime* (up to about 80% of capacity), deteriorating rapidly as the storage trends toward full. So add in *two* conversions there too -- and now only 20.2% of the input energy is available.

If the motor is a high-efficiency hubbed PM/PWM DC design you might reach 85% efficiency with it in typical use (you can do better under certain limited circumstances, but not over the entire operating range.) Take ~3-4% more off that number if you need a reduction gearbox in there.

In other words about 17% of the energy content in the natural gas winds up moving the car.

A current ICE can and does get into the 30ish% range end-to-end, 5% or so higher if the engine is diesel.

In other words the ICE wins the efficiency race by nearly a factor of TWO since it undergoes only one conversion from input fuel and while its thermal efficiency sucks (the maximum theoretical efficiency of a heat engine is the delta between combustion and exhaust in Kelvin) it has only driveline losses from there.

Oh, and we haven't talked about cost yet. Reformers are not cheap and neither are compressors. The storage tank in the vehicle is FAR more expensive than a gas tank and MUCH more dangerous in a serious accident in that if it loses structural integrity the released energy (assuming the hydrogen does not ignite!) of a full tank is going to be measured in "sticks of dynamite equivalent." Finally, fuel cells *themselves* are expensive as they're full of rare-earth metals.

You're far better off compressing the CH4 and using it in a conventional engine on an end-to-end efficiency basis. It's impractical for personal vehicles, however, due to the tank size required to get reasonable range. It is, however, being used with great success for local delivery vehicles and transit buses where tank size and range are not much of an issue.

In short we don't use liquid hydrocarbons for transportation fuel because we're pigs. We use them because it's very hard to find something that will fit 110,000 BTUs (or thereabouts) of energy in a one-liquid gallon sized container that masses six to eight pounds and requires only another few pounds of container to safely hold it, all of which must be accelerated by the vehicle and thus damages that vehicle's overall efficiency.

You cannot cheat the laws of thermodynamics.
 
#8 ·
UPDATED: One line engineered drawing

Car and Driver just posted a blog reaffirming Mazda's direction for the Wankel motor as a generator for a hybrid vehicle. Included in the blog is a one line drawing of the driveline, showing an electric motor up front and the Wankel motor/generator mounted in the back. Interestingly enough, Mazda has also pushed back the debut of a hybrid until 2022 in the form of a subcompact. This seems to dispel any notions that Mazda and Toyota might be working together on a hybrid platform for Mazda which begs the question: What benefit is Mazda getting from Toyota on their joint-venture?
 
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