Gazoline injection at TDC 2

[spigor]

Why don't they make a gazoline engine that compresses only air and uses fuel injection at TDC (top dead center), just like in diesel? Looks tempting - high compression, better efficiency and so on. They even made a HCCI engine with lots of control problems that didn't stop them, so there must be a fundamental reason not to inject the gazoline at TDC. I tried very hard to find that reason on the web, but no result. Could anyone tell me? Thanks a lot in advance.

 

[Tmoose]

I think gasoline "burns slower" (has a greater delay period, related to octane rating) than diesel, and diesel engines have to start injection maybe 10 degrees BTDC, so burning gasoline with compression ignition would have to be injected even earlier than that.
Gasoline is not the right fuel for compression ignition

 

[ivymike]

Gasoline is not the right fuel for compression ignition by design.

 

[TDIMeister]

Unless operating in HCCI mode...

 

[ivymike]

yeah, so here's one for y'all... if you were to do what the OP suggests, wouldn't that be a diesel cycle, even if not run on "diesel" fuel? If it's a diesel cycle, why would one want to run a fuel other than one designed for the cycle, and so named?

 

[BrianPetersen]

Presuming that one still wants to use spark ignition, gasoline only ignites by spark once vaporized, and it needs time to do that, so at best the injection would be late in the compression stroke but not actually at TDC.

And, back when there was a thought of using gasoline-direct-injection in lean-burn mode, some of them *would* inject sometime into the compression stroke under some operating conditions.

But, emission regulations (primarily NOx) killed the idea of lean-burn gasoline direct injection. If you are going to use stoichiometric direct-injection, there is little purpose to injecting it late. Might as well inject it early and give it more time to vaporize.

You COULD theoretically still use lean-burn gasoline direct injection, if you use the newer de-NOx catalysts that the diesels are doing. But, that's expensive. If you are going to make a gasoline engine as expensive as a diesel engine because of the exhaust aftertreatment, you might as well go all the way and use a diesel engine running on diesel fuel.

 

[Pud]

Is not this similar to what the op is writing about?

http://www.audi.co.uk/audi/uk/en2/new_cars/Technology/engine/fsi.html

I drive an Audi with an FSI engine in it and it will pull cleanly from 1200rpm in top gear, foot to the floor. Wouldn't consider it particularly economical though.

Harry

http://www.tynevalleyplastics.co.uk

 

[JSteve2]

While diesel is optimized for direct injection, not all of the barrel of oil "wants" to be diesel. The light fractions are currently used as gasoline, and conversion of that portion to diesel would increase the cost signficantly (in some sense, it would be synthetic fuel). Some manufacturers are toying with even producing direct injected gasoline engines (Mazda, for example). I'm sure they are struggling with all the listed challenges, including NOx. Mazda is using a spark still, so I'm sure many of their issues are around getting the benefits of high compression while still delaying combustion until the spark hits.

 

[BrianPetersen]

"Toying with producing" was several years ago. All VW/Audi current production engines with the "FSI", "TFSI", and "TSI" monikers are direct-injection gasoline engines, and the early ones had lean burn but that has since been abandoned. The VW 2.0T in the GTI, GLI, Passat, Eos, Tiguan is the most common one we see in North America.

All Mitsubishi current production engines with "GDI" are direct injection - mostly outside North America. I think they were first to production with this.

There is a version of the GM Ecotec 4-cylinder with direct injection in current production.

The BMW 335i uses gasoline direct injection.

Honda, Mercedes, Mazda are others that I can think of, off the top of my head, that have current-production direct injection gasoline engines although not all are in North America.

 

[waross]

While diesel is optimized for direct injection, not all of the barrel of oil "wants" to be diesel. The light fractions are currently used as gasoline, and conversion of that portion to diesel would increase the cost signficantly (in some sense, it would be synthetic fuel).

As more and more heavy oil and bitumen sourced synthetic crude are coming online, this is becoming less of an issue.
Heavy oils have to be "upgraded " to diesel and upgraded further to produce gasoline.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[TDIMeister]

Almost all new gasoline engine development projects outside of BRIC countries are focusing on direct injection, and most are of the homogeneous (early injection) variety.

The type of DI used in the Mitsubishi GDI and VAG FSI are termed first-generation side-injector wall- or air-guided systems. BMW is the first I know of to put the state-of-the-art spray-guided system with centrally-mounted piezo injectors (not too far removed from Diesels!) to series production in its "High Precision Direct Injection" system. It's really quite a neat piece of work. Hardly a European OEM doesn't have a development project for spray-guided DI, even DI pioneer VAG has such a program to follow their highly successful (T-)FSI. Why I wax lyrical about SG-DI here is because the BMW system has up to three separate injection events per stroke, with the last one coming very close to TDC and spark plug firing. Contrary to intuition, this last, very late injection drastically lowers soot emissions (yes, read up people, gasoline engines DO produce soot, DI engines especially more than PFI ones!).