Atkinson to Otto Cycle

The number one operation consideration of the Prius is emissions. The number two operation consideration is fuel efficiency.

(This excludes things like repair-ability and durability of course.)

So the fact that converting to Otto cycle would damage efficiency mitigates against making it available.

Of course the recent roll-back of CAFE standards may change the algebra of this decision.

 

From TOYOTA:

"The C-HR's turbo petrol engine alternates between the conventional Otto cycle in normal driving and the highly efficient Atkinson cycle ...".

Yes, it's possible - from what I read from the motoring journalists with the C-HR, it's relatively gutless and needs every bit of help it can get.

With PRIUS, it's about balance - as bbald123 says, power isn't the aim with PRIUS, rather emissions and economy.

 

Having two modes of operation requires a compromise in the physical compression ratio - it must not be so large that you get detonation when using full power (Otto cycle) but must be large enough to get beneficial improvement in Atkinson mode. The ignition timing may also be compromised.

My previous car, a Mazda 3 had a similar feature (although not the port injection) and it did give impressive highway fuel economy although city driving was still not great.

Having Direct Injection (as in the Tacoma) seems to allow more flexibility - the Mazda ran at ~13:1 compression even with 87 octane fuel. The Tacoma has 11.5:1 ratio which is higher than usual.

If a Prius ran with this mode the compression ratio would have to be reduced from its mechanical 13:1 ratio and the efficiency compromised.

I was a bit disappointed that Toyota did not use direct injection in the 4th Gen Prius - maybe they found that any efficiency improvement wasn't worth the cost and possible reliability issues.

For decades engine designers have attempted to design variable compression ratio engines which is another way of increasing efficiency but so far none have been successful. Renault is supposedly attempting to bring one to market : (Variable Compression Ratio Coming to New Infiniti Four-Cylinder Turbo – News – Car and Driver | Car and Driver Blog)

kevin

 

The C-HR is heavier than the Prius I believe, but has the same drivetrain. It might need a little extra power from the ICE, whereas the Prius can satisfy that need through the battery and motor alone.

 

HPDI setup will require
much more regen from engine....this is a problem with HPDI ...just look any car that has this muffler is black...also there will be issue with intake valve buildup....
This approach is just another band aid to improve MPG where emission will get more complex and will cost more
There are many benefits in HPDI
higher compression
Tighter timing
More control over fuel air mixture
But it will require much hotter combustion chamber that will need more expensive materials to withstand
It will need similar setup what diesel engine have today....
Then engineer is back to square 1
Diesel is much more efficient than gas
And in near future catalytic converters will employ electric heating that will make gas engines go a way...
Biggest issues diesel engines fight now is using fuel to activate regeneration....
Once this technology is ready...Diesel engine will become main focus to all car company's
Posted via the PriusChat mobile app.

 

i can't really say the prius needs anything until someone comes along with a better product for the same money.

 

Definitely agree....


Posted via the PriusChat mobile app.

 

Yes, I wondered about that - a lot has been spent on Petrol (gasoline) engines over the last decade making them more efficient, but diesel seemed to have nothing much in the way of "news" since they put particulate filters on them. But Diesel is appreciably more efficient, but research on them seems to have stalled.

I've had 2 diesel cars (without particulate filters), great economy, appreciably better than equivalent cars, (though the PRIUS is better still). My Diesel FOCUS averaged 5.4 l/100km (43.6 US MPG), my wife's PETROL Focus averaged 9l/100km (26.1 MPG US).

 

The Prius variable valve mechanism doesn't have adjustment range, at least without sacrificing quite a bit of its Atkinson range. Unless it has some newer and better variable valve system, I strongly suspect that the Tacoma doesn't go nearly as deeply in Atkinson-like operation.

 

The problem with diesels is the high temperature at combustion. This leads to increased NOx emissions as VW's experience has proved. I imagine this would be a problem to be solved for any hot burning technology.

 

Yup. And the EU has started looking at the decision they made to promote diesels decades ago.

 

Or the wider valve timing range for Otto to Atkinson was just deemed not worth the cost on a Prius after adding all the other new improvements. With a large battery pack on hand, an electric supercharger might be the better option in terms of cost. But that would still add a few hundred to the base MSRP.

The European press raved over GM's new 1.6L diesel, and the US is actually getting it in the Cruze now, and the Equinox next year. The challenge for diesels is in improving emissions. Diesels can be made that match the Prius in terms of EPA emissions, but the cost and maybe the reliability still needs to improve. With the US having the strictest emissions for personal cars, and being a small market for such with diesel engines, the incentive for companies to develop these clean diesels isn't as great as for advances in gasoline cars.

There is some work going on. The aforementioned 1.6L had the North American market in mind since its beginning, and isn't just an European diesel adapted to US regulations. Cummins is also working on a small 4 cylinder(2 to 2.5 liter range) diesel with very clean emissions. The work is with Nissan, and it should appear in the Frontier in the next few years.

Then emission improvements alone just don't get the press that other car technology advances does. The next Mazda3 getting HCCI(gasoline with compression ignition like a diesel) is just sexier than this diesel qualifies for the EPA's cleanest emission bin.

Mazda's SkyActiv diesels are lower compression for lower temperatures, but fuel to air mixture is still a big factor in NOx formation. Lean burning gasoline engines were once common, but disappeared when NOx limits got tighter. The federal Insight was the last one I believe, and it add a NOx trap like diesels now have.

Diesels were always lean burn. The reason why they are is part of the actual 'secret' to their better efficiency. In a basic gasoline engine, the output is controlled by controlling the amount of oxygen available for burning fuel. This is done by the throttle plate. Fully open, air rushes in, providing the most amount of oxygen possible, which allows the most amount of fuel to be burned within the cylinder.

Closing the throttle plate reduces the amount of air coming in. The amount of fuel injected is reduced since the lower oxygen amount would result in fuel being unburnt and wasted. With each revolution, the engine is still trying to suck in the maximum amount of air it can take. This means the air pressure behind a partially closed throttle plate drops, forming a partial vacuum. In turn, the vacuum pulls on the descending pistons on the intake stroke. The pistons working against that vacuum is major part of what is referred to as pumping losses; these reduce the engine's efficiency.

Diesel engines don't control airflow to control output, they control fuel flow. This means there is no throttle plate on them, and the engines gulp down their full volume of air at all engine speeds. This means no vacuum forms to add to pumping losses. Once engineers started using the vacuum formed in gas engines to run accessories and power boost the brakes, they had to install a vacuum pump for such on diesels. It also means that there is extra oxygen in the cylinder at all times it isn't running at full output. That oxygen that doesn't burn with the fuel, burns with the nitrogen in the air to form NOx. The other part of the diesel's efficiency secret is high compression ratios. This allows more energy to be extracted from the burning fuel. The higher temperatures formed by the higher compression also increases the amount of NOx formed.

In some ares of the US, the per gallon cost for diesel is close to that of regular gasoline. On the East Coast, in is on par with premium gas. Emission control technology for diesels is still young compared to that of gasoline engines; getting them clean with low cost and reliability is just a factor of time. Buying and adding DEF for a SCR system is another maintenance item and cost for a diesel, but really one inline with adding windshield fluid. The perception of it is a negative to a diesel sale, but the big hurdle is paying more for fuel after paying more for the engine.