Mazda high compreesion gas engine, low comp. diesel

[SBjet]

Mazda has a really high compression gas engine, with a really low compression diesel engine.
http://www.mazda.com/mazdaspirit/skyactiv/engine/skyactiv-g.html

Now they are saying they are able to do this because they are doing a great job of scavenging the exhaust. Well I always thought you could easily scavenge more exhaust by shortening the exhaust pipe, but that could lead to too much heat/lean and would start burning up your engine parts; exhaust valves, pistons.

And what about the low compression diesel? If high compression makes power, why do you want to lower it?

 

[DaveH]

Mazda has a really high compression gas engine, with a really low compression diesel engine.
http://www.mazda.com/mazdaspirit/skyactiv/engine/skyactiv-g.html

Now they are saying they are able to do this because they are doing a great job of scavenging the exhaust. Well I always thought you could easily scavenge more exhaust by shortening the exhaust pipe, but that could lead to too much heat/lean and would start burning up your engine parts; exhaust valves, pistons.

And what about the low compression diesel? If high compression makes power, why do you want to lower it?

hi compression NA engines are nothing new (think racing engine) but were not very feasible becasue of the high octane fuel required. to me, what has changed all this, even more than what mazda is touting with their exhaust technology is the use of direct injection. the reason this is so big is because knock (pre-ignition) is eliminated. it works like this....in a carb or port injected engine, the fuel goes into the cylinder during the intake stroke and is then compressed. this introduces significant heat to the mixture and the fuel must resist burning or 'exploding' and wait for the spark event to ignite. this is no small feat to achieve. this is where higher octane fuel comes in... it has nothing to do with high octane fuel on its own "making more power"......its all about resisting combustion starting on its own (early) creating knock. a DI engine that puts fuel the cylinder directly at or near TDC eleiminates the need for the fuel to undergo the compression stroke and absorb less heat and the milisecond of time it spends beofre the ignition event all but eliminate knock.

as for the claims made on the low compression diesel engine, it still uses a turbo charger, so cylinder pressures can go up substantially. they are using new injectors to better distribute and atomzse fuel for better emission and performance under "light load" engine operation to maximize efficiency and emissions.

 

[Bigmutha]

We bought a 2014 Mazda6 with the Skyactive motor. The car runs great and gets 38 mpg on the highway.

 

[Froggystyle]

hi compression NA engines are nothing new (think racing engine) but were not very feasible becasue of the high octane fuel required. to me, what has changed all this, even more than what mazda is touting with their exhaust technology is the use of direct injection. the reason this is so big is because knock (pre-ignition) is eliminated. it works like this....in a carb or port injected engine, the fuel goes into the cylinder during the intake stroke and is then compressed. this introduces significant heat to the mixture and the fuel must resist burning or 'exploding' and wait for the spark event to ignite. this is no small feat to achieve. this is where higher octane fuel comes in... it has nothing to do with high octane fuel on its own "making more power"......its all about resisting combustion starting on its own (early) creating knock. a DI engine that puts fuel the cylinder directly at or near TDC eleiminates the need for the fuel to undergo the compression stroke and absorb less heat and the milisecond of time it spends beofre the ignition event all but eliminate knock.

as for the claims made on the low compression diesel engine, it still uses a turbo charger, so cylinder pressures can go up substantially. they are using new injectors to better distribute and atomzse fuel for better emission and performance under "light load" engine operation to maximize efficiency and emissions.

What a great explanation. I am just surprised that the fuel can get in there that fast, that often. Seems like you would only have milliseconds to have it completely fill the chamber.

What kind of pressures are those injection systems running?

 

[DaveH]

What a great explanation. I am just surprised that the fuel can get in there that fast, that often. Seems like you would only have milliseconds to have it completely fill the chamber.

What kind of pressures are those injection systems running?

it varies from engine to engine....but generally the fuel pressure is in the THOUSANDS of pounds..... as you noted, the time to do this is so small that the fuel must be introduced very quickly, the high fuel pressure overcomes this, as well as overcoming cylinder pressure it has to inject into.

the "headache" with DI is that for injectors to open at these split second times against wicked fuel pressure, the injectors require very high voltages to operate (compared against a DC 12vovlt system) and the associated electronics in the vehicle computer to allow for this. Also, becasue the injector is now exposed to the combustion event, early DI systems were having issues with carbon deposits building up on the injector tip clogging operation.

 

[Racey]

What a great explanation. I am just surprised that the fuel can get in there that fast, that often. Seems like you would only have milliseconds to have it completely fill the chamber.

What kind of pressures are those injection systems running?

Diesels have been doing this for well, decades, nearly a century... direct injection that is, it's just now coming of it's own in gas motors, though there have been DI gasoline engines in production vehicles for probably 10 years now, they are getting more and more popular.

But yeah, it's a shit load of fuel to spray in a very short time period. at 6,000rpm you only have 20 milliseconds for the engine to perform a complete cycle (2 rotations of the crank) meaning that each stroke of the piston is only 5 milliseconds, but you can't spray fuel for the entire 180 degrees of a stroke, more like probably 45-90 degrees would be my guess of a maximum useful spray during burn duration. so that is closer to around 1-2ms of useful time. I would imagine that they aren't dumping 100% of the fuel all at once and then lighting it, but rather they are spraying into the cylinder just before lighting, and then continuing to spray through the first part of the stroke. But i could be wrong on that, it's just an assumption of how i imagine it working. Edit: Apparently they only inject under compression stroke during extreme light load, and decel, under load conditions they are spraying the fuel into the chamber directly after the exhaust valve closes, and during the intake stroke.

The technology lies in the injectors, since the burst durations are very short the injectors need to be extremely consistent. Good port injectors (normal injectors) start having trouble with consistency below 1ms of pulse as the time to fully open and close eats into a good amount of that pulse length, Normal idle injector duration is around 1.5 to 2.0ms, and at the top end under load you have a useful pulse duration of 85% of the total engine cycle (the injector needs at least 15% off time to no burn itself out, ie duty cycle) So at 6,000 you have up to 17ms of useful spray time, or around 600 degrees of crank rotation, to safely deliver all the fuel needed. In other words way more time than with direct injection.

Edit: just as 10 or so years ago when the aftermarket community started going from carbs over to stand alone port EFI, we will see the same thing happen again as we make the switch from port to direct injection because of all of it's benefits.

 

[pronstar]

Keep in mind that diesels also run multiple injector events - seven or so in some cases - at rail pressures well over 20k psi. So the injector technology is there.

Direct injection for gas is about an order of magnitude less - around 2-3,000 psi.

Big advantage to low-compression diesel is reduced pumping losses. Having the turbo increase in-cylinder pressure is more efficient than doing it mechanically.

 

[Racey]

Keep in mind that diesels also run multiple injector events - seven or so in some cases - at rail pressures well over 20k psi. So the injector technology is there.

Direct injection for gas is about an order of magnitude less - around 2-3,000 psi.

Big advantage to low-compression diesel is reduced pumping losses. Having the turbo increase in-cylinder pressure is more efficient than doing it mechanically.

on the other hand you give up "expansion ratio" used to convert expanding gas pressure into mechanical energy. There is always a balancing point there, the ad absurdum would be imagining a motor with 2:1 compression but running like 200psi of boost, with only room to double in size most of the charge's energy goes out the exhaust pipe as heat, instead of being put to mechanical use.

 

[U4ia]

Keep in mind that diesels also run multiple injector events - seven or so in some cases - at rail pressures well over 20k psi. So the injector technology is there.

Direct injection for gas is about an order of magnitude less - around 2-3,000 psi.

Big advantage to low-compression diesel is reduced pumping losses. Having the turbo increase in-cylinder pressure is more efficient than doing it mechanically.

Another big advantage of lower compression diesel is a lighter engine. Less pressure means a lighter block, a lighter block means a diesel can be just as light as a gas engine which results in an even larger diesel advantage in fuel economy. I assume the ultimate goal is a diesel that is the same weight and nearly the same cost as a gas engine, hell the diesel could eventually be cheaper.....

 

[squeezer]

Diesels have been doing this for well, decades, nearly a century... direct injection that is, it's just now coming of it's own in gas motors, though there have been DI gasoline engines in production vehicles for probably 10 years now, they are getting more and more popular.

First production DI gasoline engine was in 1954...

 

[pronstar]

on the other hand you give up "expansion ratio" used to convert expanding gas pressure into mechanical energy. There is always a balancing point there, the ad absurdum would be imagining a motor with 2:1 compression but running like 200psi of boost, with only room to double in size most of the charge's energy goes out the exhaust pipe as heat, instead of being put to mechanical use.

True, and that expansion ratio is why Atkinson Cycle gas engine is why they're around 12% more efficient than Otto Cycle engine...and also why they have lower output power density.

Interestingly, Mazda took the lead with production Atkinson Cycle engines about 20 years ago, they slapped a supercharger on it which effectively made it a Miller Cycle engine.

 

[pronstar]

First production DI gasoline engine was in 1954...

First, versus widespread, are very different.

The Mercedes-Benz 300SL had fuel injection in the 50's, but carbs were still widespread until the 80's.

 

[TPC]

I read someplace the Duramax compresson has been lowered over the years and it's made them much quieter and smoother running.

 

[OCMerrill]

First, versus widespread, are very different.

The Mercedes-Benz 300SL had fuel injection in the 50's, but carbs were still widespread until the 80's.

They better make this stuff reliable. My 08 HHR SS with the Direct Injected Ecotec has lost it's HPFP 2x. 1st under warranty and the second we just paid for. It was just under $650.

They only spray about 150 psi at idle and up to 2500 psi full throttle. Not even close to Diesel pressures. Our Cummins common rail wont even fire if the RPS does not see 2500 min pressure.

 

[pronstar]

They better make this stuff reliable. My 08 HHR SS with the Direct Injected Ecotec has lost it's HPFP 2x. 1st under warranty and the second we just paid for. It was just under $650.

They only spray about 150 psi at idle and up to 2500 psi full throttle. Not even close to Diesel pressures. Our Cummins common rail wont even fire if the RPS does not see 2500 min pressure.

The first generations of high-pressure diesel injectors were troublesome.
Ask early Duramax owners...

They get better and better, though.
I think I posted this before, but IMHO it's amazing these injectors last even 5 minutes.
The tip is in the harsh combustion chamber, you're getting 3 - 7 injector events per stroke, and you're operating at well over 20,000 psi.

 

[Racey]

True, and that expansion ratio is why Atkinson Cycle gas engine is why they're around 12% more efficient than Otto Cycle engine...and also why they have lower output power density.

Interestingly, Mazda took the lead with production Atkinson Cycle engines about 20 years ago, they slapped a supercharger on it which effectively made it a Miller Cycle engine.

Yep the valve timing, especially on the intake is the important part, let some of the charge be pushed back into the intake without sacrificing the compression ratio for power extraction. Static vs Dynamic compression numbers.

 

[OCMerrill]

The first generations of high-pressure diesel injectors were troublesome.
Ask early Duramax owners...

They get better and better, though.
I think I posted this before, but IMHO it's amazing these injectors last even 5 minutes.
The tip is in the harsh combustion chamber, you're getting 3 - 7 injector events per stroke, and you're operating at well over 20,000 psi.

Some of these new engines coming out in the new cars and 1/2 ton trucks spin to 4500 rpm. A multi event at 4500 rpm sounds crazy but they are doing it.

My 04 Only has two events. 174k no injector drama...yikes, yet.


Duramax = 8 at how much each?