Featured Mazda competes for thermal efficiency against Toyota with Skyactiv-Z

I believe that generally it just becomes too expensive to make a diesel-hybrid in most cases, so less R&D to make it ultra efficient. And with the general lack of interest in small passenger diesel cars, they would be less profitable, even if they did get substantially better fuel mileage, as people willing to spend that kind of money probably would just get an EV at this point.

One thing I do wonder is, why can't we have an atkinson cycle diesel? How efficient would that be?

Another point is that maybe we're looking at the wrong kind of vehicle. (I see @hill beat me to this point.) While my fondest memories of driving were in my ol' 1985 VW diesel Golf, the place where a diesel hybrid could really shine is in not in little passenger vehicles, but in commercial vehicles. Yesterday I fueled my 60,000 lb GVRW water truck. It was over $300. Ouch! A lot of that fuel was wasted every time I had to brake as I drive around, which happens a lot. I remember one day hitting every single stop light in red as I went through town. Last week I drove a semi over the mountain passes here down into the city and averaged just above 6mpg. A lot of that fuel's energy was wasted in braking down those long steep grades.

While full electric could be the way to go someday, all of our commercial equipment needs to be able to get to places hundreds of miles from here, which makes pure battery electric trucks prohibitive right now. (There are Ford LIghtnings in our fleet and a few have ran out of juice while on the road necessitating lengthy charging times.) Diesel hybrids could be a good stop-gap between no regenerative braking and full battery electric.

I don't see gasoline-hybrid commercial vehicles ever being a solution.

 

There have been decades of myths around lean burn, so let me explain.

The hottest burning AFR is stoichiometric, pure and simple. Stoichiometric is too hot at full throttle. You see, as you go leaner or richer from stoichiometric the combustion temperature lowers. What makes an engine "too lean" is one that doesn't enrich enough at full throttle, as gasoline engines typically use a rich mix for detonation mitigation at high loads. That "too lean" can be stoichiometric or just above or just below it.

Another technique that has been used to avoid detonation is actually running much leaner than stoichiometric at full throttle. Contrary to popular belief, it can and has been done before and has worked just fine without causing detonation. But the main problem is this doesn't give you full power, whereas a rich mix does. And the second is that it makes it hard to get rid of NOx.

You actually get a lot more NOx formation in a stoichiometric, or slightly leaner than stoichiometric homogeneous gasoline engine than a true lean burning engine, including diesel engines, as stoichiometric engines burn much hotter. But the homogeneous mixture means there's enough left-over fuel to use to react with the NOx and reduce it down to levels lower than a lean burning engine.

Adding EGR further reduces the initial formation of NOx. EGR and lean burn are very similar, in that you are adding a inert gas to the mix (air without fuel is inert too), which both give you benefits in reducing pumping losses. You'll also get similar NOx formation in both a lean burning and an EGR fed stoichiometric engine. The problem is that the EGR fed engine will tend to do what stoichiometric engines do and have lean and rich pockets that don't burn up all the fuel leading to lower efficiency, whereas the lean burning engine will have little to no fuel left over that can react to the NOx that formed.

And this applies whether it's a gasoline engine, a diesel engine, a propane engine, a CNG engine, etc. The only difference is we can't make a homogeneously mixed stoichiometric diesel engine, so all diesel engines are lean burning. But at any rate, lean burning gasoline engines and diesel engines have many of the same advantages and disadvantages.

 

Except that it's not a myth, based on my experience flying airplanes. There is a mixture control, which controls the fuel/air ratio for different altitudes, and a cylinder head temperature gauge (in the planes I have flown). The procedure is to lean the mixture as you climb, limiting the leaning mixture by the cylinder head temperature, which increases with the leaner mixture.

I saw the video declaring it a myth, but it was using specious arguments, not unlike a flat earth dissertation. A rich mixture cools the combustion chamber by vaporizing unburned fuel.

 

And what were your AFRs exactly? 15:1 is still very close to stoichiometric.

A rich mix does cool the combustion chamber by vaporizing unburned fuel. A stoichiometric AFR doesn't have much of that effect. Slightly leaner and you also are still very hot. But down below 16:1 and the temps cool off as the air is inert and only cools the combustion. It's the same principal as EGR, which also cools the combustion event.

I also did experiments of this type in a 1972 air-cooled VW Beetle. I could run as low as 17:1 and saw both lower head temps as well lower as oil temps. I drove all over as a daily driver with it setup to rise to 12:1 at full throttle and lower to around 16:1 to 17:1 at cruise. I had zero detonation issues and zero overheating issues.

 

Air without fuel is around 21% oxygen, which pretty much nobody calls inert.

 

Thanks for the English lesson. And this helps or hinders lean burn thermal efficiency and emissions how?

 

It maybe gives you an opportunity to offer a better explanation of what's similar and different between EGR and lean burn, because one of those does introduce inert gas into the mixture (thus does not change the fuel:oxygen mixture, only allows less of it into the cylinder) and that's not what's happening in the other.

 

Modern, microprocessor controlled systems can alter ignition and valve timing to compensate for premature ignition and detonation. Don't ask me how, because I don't know.

 

It seemed diesel engines had the best potential when it came to alternative fuels. Especially when considering replacement potential for the current fleet. Exxon's process for e-gasoline may have shifted that.

E-fuels start with a reversed reaction of the one in steam reformation of natural gas. It takes water and CO2, and green electricity to push them along into methane. Which is a clean burning fuel on its own, which makes it popular for trucks and buses in metro areas. For cars and longer distance trucks, it has the same problem as hydrogen. The high pressure tanks take up too much space. Converted cars have half the range at best of the gasoline model, while losing trunk space.

So the green e-methane is converted to a liquid; either methanol or a light syn-crude. Methanol beyond low blends with gasoline requires modified engines. Refining the syn-crude into diesel doesn't take much; more effort needed to make gasoline from it. Exxon has a way of converting methanol to a direct gasoline replacement though. It is what Porsche is using at their e-petrol plant in Chile.

Compare your Prius to a power hybrid Lexus like the LC. The majority of the PHEV diesels are power hybrids, so will look bad in comparison to an efficiency hybrid. Then most or all the diesel hybrids available are mild hybrids, which means gains per car are lower.

The PNGV prototypes were full hybrid diesels. They were in the 70 to 80 mpg range.

So they'll match their 10 year old diesel.

The two air mixture zones were for the consistent ignition of the high lean mixture by igniting a rich zone first. The lean ratio alone may not ignite directly with a spark. Toyota and others will likely be using this concept in their engines.

True Atkinson engines have a cam between the piston rod and crankshaft. This resulted in the power stroke being longer than the charge stroke. The deflection angle of the piston rod is also lowered, reducing internal friction of the piston sliding on the cylinder wall. Atkinsonized Otto engine adjust intake valve openings to mimic the former. They don't benefit from the latter. In the end, both are trading torque and power for fuel efficiency. Without an electric motor or supercharger, they don't make great car engines.

Diesel engines are already have high compression ratios; higher than gasoline engines. So the long power stroke is already there. Seeing how they use compression to ignite the fuel, reducing the compression ratio of the charge stroke could lead to issues. They already see much of the efficiency benefits of the Atkinson, plus that of lean burn and no pumping losses from the lack of a throttle plate.

The lower exhaust temps can also reduce the catalytic converter efficiency.

If lean gasoline engines can't control NOx formation in the cylinder, then the 3 way cat is going to need help using emission tech from diesels. Mazda might have got it working in the SkyActiv-X, but the efficiency gains they first claimed ended up being half. It also never came to the US, which has stricter NOx limits.

A common chart showing emissions in relation to air/fuel ratio.

It is a myth. One that started when carburetors where the only option on cars. The factory setting for the fuel trim wasn't at 14.7:1, it was slightly rich. People would then turn the screw to lean the mix, but they were stopping at stoichiometric and not getting into actual lean ratios. When things broke from too much heat, they blamed the fuel trim as being a lean mix, when it was because they removed the cylinder cooling of a slightly rich mix.

As for planes, "Both automotive and aircraft engine carburetors contain devices that effect automatic mixture ratio changes. However, these devices function in relation to power ranges and are not sensitive to air density changes."
The Fuel Air Mixture - AOPA

As altitude increases, air density drops, which leads to less air in the engine cylinder. The air/fuel ratio becomes too rich, so you reduce the fuel trim. However, you did not make the ratio lean. With less air in the cylinder, less fuel is needed for the 14.7:1 ratio.


The chart doesn't give the stoichiometric point, but it clearly shows that cylinder temps drop as the air ratio gets richer, and more so with leaner. Temperatures increasing with reduction in fuel trim, means the ratio is returning to 14.7:1 from a rich mix.

 

There is no special linkage mechanism in the Toyota Atkinson cycle engine. The effect is achieved with valve timing. I know this because I looked it up at one time.

 

Trollbait explained exactly that in the post immediately preceding yours. Did you mean to suggest otherwise?

 

Simple: gas that doesn't burn or cause any reaction. Oxygen without fuel is, relatively speaking, inert.

What do you cool your radiator with? Air! How does air cool if it's not "inert". Because it's cooler than the radiator and doesn't have any fuel to react with, which causes heat transfer.

It's the same in a lean burning engine. There's not enough fuel to burn all the oxygen out of the air. Therefore that air and oxygen are inert and displace the combustion gases, causing lower combustion temps. You are literally burning less fuel in the same volume of ambient temperature (at ambient air pressure) gas.

That's the same as EGR, gas that doesn't have any fuel to burn with. Except EGR allows for lower oxygen levels and higher unburned fuel levels in the exhaust which is ideal for NOx reduction in a catalytic converter.

The bottom lne is burning less fuel in essentially the same volume and mass of a gaseous mixture, be it pure air or air and recirculated exhaust gas, produces less heat. There's no way that burning less and less fuel is going to get hotter and hotter. That would break the laws of physics of true.

All the decades of talk about lean being hotter is based on leaning down to around stoichiometric, and not about leaning down past that. The leaner you go from stoichiometric the cooler the exhaust and engine will be. The laws of physics dictate that very clearly.

 

Using 'oxygen' and 'inert' in the same breath is sort of, well, breathtaking.

Air cools by having thermal mass and being blown through a radiator. (And, well, its 21% oxygen content is too low to spontaneously combust the squashed bugs in the fins.)

Probably there was / is some way you can explain the point that you want to make, without having to invent a personal meaning of 'inert'.

 

What exactly is your issue?

 

.

 

I was wondering why you would post

directly after Trollbait's post that never said there was, and why you would put

directly after Trollbait's post that had already explained that.

It could seem as if your purpose was to somehow misrepresent Trollbait's post as having said something it didn't, and also not said something it did—especially as that was the entirety of your post #50; it might look different if you had been making some larger point that those bits were just incidental to.

It wouldn't be your first time making posts on PriusChat that misrepresent other posts. I don't suppose I need to explain why that's an issue. But I didn't want to just assume that was your purpose, but rather to ask, so if you had some other explanation to give you could do that.

 

Why is this so important to you?

 

Sorry for saying that a gas that doesn't burn is inert. You can take "air without fuel is inert" as a metaphor of that works for you.

In reality, even recirculated exhaust gas is not truly inert as nitrogen will react with certain metals and cause them to burn with enough heat applied.

And that's exactly what happens in a lean burning engine. The extra air that doesn't react is just thermal mass, lower than that of the combusting fuel and oxygen.

 

gambling the criminal automotive behavior card?
the best VW card to play is over 10 years old? lol.
I'll see your decade old VW criminal Behavior
..... and call - toyo's emission non-compliance, that's WAY less than a decade old.

Office of Public Affairs | Toyota Motor Company to Pay $180 Million in Settlement for Decade-Long Noncompliance with Clean Air Act Reporting Requirements | United States Department of Justice
would have been easier to acknowledge diesel can be just as clean or dirty as a gasser. Pride; - it gets in the way of learning sometimes

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