Doin what they say cant be done.

[cRiPpLe_rOoStEr]

Originally Posted by Sparkeysmall

How does increasing the compression reduce economy? By that logic a prius would be running 4:1. But it uses the atkinson cycle. And from what i found in one min of googleing in the NZ engine, it uses a 13:1 static but with the cams gets dropped to an effective 8:1. So why would they squeeze the 1.5l engine physical size with a smaller effective size because of the atkinson cycle to such a high ratio? Because the greater expansion from spark to exhaust stroke allows better use of the heat generated.

Now. Compairing any atkinson engine to an otto is like apples and oranges.

So what about a typical non-hybrid, otto cycled car. From the multiple modern econoboxes i googled they all where around 10.5:1. Pretty much the max you could use while pumping regular gas. Albiet with a very tiny camshaft.

Anyway. Squeezing a large mixture into a tiny mixture increases heat generated by compression alone. And a hotter mix burns faster, producing more power earlier on the down stroke, as well as a more complete burn (warm and hot air intakes for instance. (Ya ya "smaller" engine, less dense, less parastic losses)). So with higher heat generated from the same amount of air and fuel, more efficiency. Why compress the fuel at all if just igniting it at ambient pressure makes more power than compressing it. Explode a small box vs a gymnaisium with the same amount of fuel and air.

Lower compression sometimes can actually make sense, since it's safer to run leaner and free of knock with a lower octane rating gas, even if that would be at the expense of some power and torque. There are also fewer pumping losses and heat rejection. Sure, there's some point when decreasing too much the compression ratio is not going to lead to an efficiency improvement, and the same goes for a higher compression ratio. For modern engines fitted with direct injection, Mazda has found the sweet spot to be around 13:1 to 14:1.

[niky]

Originally Posted by cRiPpLe_rOoStEr

Lower compression sometimes can actually make sense, since it's safer to run leaner and free of knock with a lower octane rating gas, even if that would be at the expense of some power and torque. There are also fewer pumping losses and heat rejection. Sure, there's some point when decreasing too much the compression ratio is not going to lead to an efficiency improvement, and the same goes for a higher compression ratio. For modern engines fitted with direct injection, Mazda has found the sweet spot to be around 13:1 to 14:1.

Mazda's going even higher.

They reckon they've got the compression ignition problem licked, so the next generation of SkyActiv is going straight to 18:1.

On gasoline.

Engineers tell me they'll still have spark plugs for cold start, but they shouldn't need them. Even better, injection pressure should be less than current common rail diesels, which means a cheaper fuel system and less issues.

I hope.

[cRiPpLe_rOoStEr]

Originally Posted by niky

Mazda's going even higher.

They reckon they've got the compression ignition problem licked, so the next generation of SkyActiv is going straight to 18:1.

On gasoline.

I could bet some sort of "multifuel" ability might be on their plans for eventual military and aeronautical applications.

Engineers tell me they'll still have spark plugs for cold start, but they shouldn't need them. Even better, injection pressure should be less than current common rail diesels, which means a cheaper fuel system and less issues.

About 10 years ago there were some rumors about either Mercedes-Benz or GM being in a hurry to release an engine with that feature, which had been evaluated by GM using some Opel Insignia and Saab 9-3 test mules and was featured in the Mercedes-Benz F700 concept. GM claimed that it could run without the spark plugs at all, but they provided a smoothier and quicker cold-start ability. Anyway, I wonder how such engines would perform with ethanol or gaseous fuels such as biomethane.

[Sparkeysmall]

Looks like ill be shopping for a mazda when the time comes

[cRiPpLe_rOoStEr]

Originally Posted by Sparkeysmall

Looks like ill be shopping for a mazda when the time comes

If either Mazda comes back to Brazil or I move to another country, I'd also consider getting one. I wonder if they're going to increase the compression ratio of their SkyActive-D engines too or if that's being done only in the gassers.

[stovie]

From what I understand you don't get the full benefit of running 100% ethanol tell you hit about 14.7-1 compression ratio!! A lot of people I've read about switching to full ethanol have got an average of 25% increase in fe from gasoline!! I'm getting ready to make belly pan and aero cover for my s10 out of sheets and epoxy here in a few weeks if you'd like to see if it would work for you??

[cRiPpLe_rOoStEr]

When my mother had a flexfuel Chevrolet Celta, she actually got a better mileage while using a higher amount of ethanol due to its improved anti-knocking properties. I don't remember its compression ratio right now, but it was quite high for a port-injection engine, and then it was quite unavoidable to get some knocking issues when sugarcane is out of season and the ethanol content of Brazilian gasoline is reduced from 25% to 20%.

[Sparkeysmall]

Originally Posted by stovie

From what I understand you don't get the full benefit of running 100% ethanol tell you hit about 14.7-1 compression ratio!! A lot of people I've read about switching to full ethanol have got an average of 25% increase in fe from gasoline!! I'm getting ready to make belly pan and aero cover for my s10 out of sheets and epoxy here in a few weeks if you'd like to see if it would work for you??

I actually just got some panels today to start cleaning up under-aero. Hopefully find some time to do that soon. 6-10's Night shift sucks.

And that 14.7:1 number sounds familiar. I tried looking for info on that and diddnt get far. Diddnt really look much either.

Anyway what originally sold me on the ethenol idea was the koenigsegg whatever that made 1100 some odd horsepower running on it. Over the 900ish it made on gas. With massive turbos of course. (Another thought, one BIG turbo with the v10. Would make towing fun... Ya twins would spool up faster. But i dont want quick boost. Something you gotta lay into for a bit to get things going. Would make econo driving easier.)

[freebeard]

Pure premium has specific combustion properties that can be tuned for, pure ethanol also has specific combustion properties. Which can be taken advantage of by increasing compression ratio and adjusting the timing.

I can see how an advanced engine might take advantage of one or the other on the fly. But when they are mixed together?

This used to be called 'neither fish nor fowl'. 14.7-1 sounds like the optimal stochastic ratio.

[cRiPpLe_rOoStEr]

Originally Posted by freebeard

Pure premium has specific combustion properties that can be tuned for, pure ethanol also has specific combustion properties. Which can be taken advantage of by increasing compression ratio and adjusting the timing.

I can see how an advanced engine might take advantage of one or the other on the fly. But when they are mixed together?

Nowadays with VVT, knock sensors, wideband O² sensors, and many other tricks, there is roughly no excuse for modern spark-ignited engines to not be able to handle gasoline, ethanol, or both mixed at any ratio. Gaseous fuels compatibility shall also not be disconsidered, as it's becoming more common in a worldwide basis.