What's the difference between DI and normal fuel injection?

[charlieMc]

What is special about the Cadillac Direct Inject engine compared to one with regular old fuel injection? Thanks.

[Ronster]

"regular old" fuel injection in this case is sequential port injection. gas is squirted into the intake manifold port towards the intake valve, hoping it mixes with the air enough to ignite evenly. DI puts the injector into the top of the combustion chamber so the intake draws in only air. fuel is squirted directly into the cylinder (hence "direct" injection) during the intake stroke and the mixing turns out to be a bit better. this along with variable valve timing and use of aluminum for the heads allows much higher compression ratios to be used. in the hfv6 di it's 11.3:1 specified with regular unleaded gas. the difference between di gas and diesel is that this engine still uses spark to ignite the fuel/air, whereas diesel injects the fuel during compression for compression-ignition which requires much higher pump pressure. even so, the DI motor requires over 1,000 psi (I believe 1700) at the injectors which means this engine has an added mechanically driven pump, on the rear of the left cylinder head. GM claims that pollution is reduced especially during warm-up.
there are several gm and holden websites that describe this in more detail. for instance see media.gm.com.
I should add that technically this is known as SIDI - spark ignition direct injection.
rg

[Mark JD]

Hey Ronster....
GREAT answer! Info I always wondered about and you explained it very well.
Thanks!

[yatesd]

Here is one picture:

[yatesd]

Very good link:

http://cars.about.com/od/thingsyoune...tinjection.htm

Direct fuel injection defined
Direct fuel injection is a fuel-delivery technology that allows gasoline engines to burn fuel more efficiently, resulting in more power, cleaner emissions, and increased fuel economy.

How direct fuel injection works
Gasoline engines work by sucking a mixture of gasoline and air into a cylinder, compressing it with a piston, and igniting it with a spark; the resulting explosion drives the piston downwards, producing power. Traditional (indirect) fuel injection systems pre-mix the gasoline and air in a chamber just outside the cylinder called the intake manifold. In a direct-injection system, the air and gasoline are not pre-mixed; air comes in via the intake manifold, while the gasoline is injected directly into the cylinder.

Advantages of direct fuel injection
Combined with ultra-precise computer management, direct injection allows more precise control over fuel metering (the amount of fuel injected) and injection timing (exactly when the fuel is introduced into the cylinder). The location of the injector also allows for a more optimal spray pattern that breaks the gasoline up into smaller droplets. The result is more complete combustion -- in other words, more power from each drop of gasoline.

Disadvantages of direct fuel injection
The primary disadvantages of direct injection engines are complexity and cost. Direct injection systems are more expensive to build because their components must be more rugged -- they handle fuel at significantly higher pressures than indirect injection systems and the injectors themselves must be able to withstand the heat and pressure of combustion inside the cylinder.

How much more powerful and efficient is direct injection?
Cadillac sells the CTS with both indirect and direct injection versions of its 3.6 liter V6 engine. The indirect engine produces 263 horsepower and 253 lb-ft of torque, while the direct version develops 304 hp and 274 lb-ft. Despite the additional power, EPA fuel economy estimates for the direct injection engine are 1 MPG higher in the city (18 MPG vs 17 MPG) and equal on the highway. Another advantage: Cadillac's direct injection engine runs on regular (87 octane) gasoline. Competing cars from Infiniti and Lexus, which use 300 hp V6 engines with indirect injection, require premium fuel.

Renewed interest in direct fuel injection
Direct injection technology has been around since the mid-20th century; however, few automakers adopted it for mass-market cars. Electronically-controlled indirect fuel injection did the job nearly as well at a significantly lower cost, and offered huge advantages over the mechanical carburetor, which was the dominant fuel delivery system until the 1980s. However, recent developments such as skyrocketing fuel prices and stricter fuel economy and emissions legislation have led many automakers to begin developing direct fuel injection systems. You can expect to see more and more cars make use of direct injection in the near future.

Diesels and direct fuel injection
Virtually all diesel engines use direct fuel injection. However, because diesels use a different process to combust their fuel (gasoline engines compress a mixture of gasoline and air and ignite it with a spark; diesels compress air only, then spray in fuel which is ignited by the heat and pressure), their injection systems differ in design and operation from gasoline direct fuel injection systems.

Cars with direct fuel injection
Audi A3
Audi R8
Audi S4
Cadillac STS
Chevrolet HHR SS
Pontiac Solstice GXP
Volkswagen Jetta GLI

[yatesd]

Nice to know I own a 304HP AWD green car:

http://www.greencarcongress.com/2007...rst_v6_di.html

[Ronster]

Originally Posted by Mark JD

Hey Ronster....
GREAT answer! Info I always wondered about and you explained it very well.
Thanks!

thanks. it's what I do. been a gearhead for over 40 years and an engineer for almost 30.
to put a historical reference on that 11.3-to-1 figure, when I was building engines in the '70s my rule of thumb was 10.0-10.5 max using premium leaded gas (street use). that was cast iron heads and fixed valve timing with a wedge chamber. my experience with a hemispherical chamber was even lower compression with premium. hemis create a lot of power but they are very dirty by nature. the quickest way to increase the efficiency and (specific) power output of an internal combustion engine is to raise the CR. so 11.3:1 allowing 87 octane is simply incredible.
rg

[Ronster]

as long as I'm at it I'd like to add some perspective regarding the engine in general. it's currently known as HFV6 for high-feature. this means it has 4 valves per cylinder, double overhead cams, variable valve timing, all aluminum construction, and other things high tech. it has really come a long way; it began as a chevy 2.8L ohv v6 near about 1980 or so, I think it first showed up in the citation (anyone remember that?). been used in camaros, s-10 pickups, cavaliers (the z-24) and its step-brother the cimarron, and heavy use in the lumina-type line (lumina, monte carlo [fwd] before 2000, olds achieva, etc), and beretta/corsica. the initial design advantage was the 60-degree bank separation which made it very compact and therefore light, and it naturally operates much smoother all compared to the typical 90-degree v6. buick had their 90-degree v6 (eventually ended up with turbos and intercooling in the grand national, beating corvettes at the time) but they had to either offset the crank journals or offset the spark timing to achieve some semblance of smoothness. more recently this 3800-series was used in the impala/monte-carlo line. 90-degree v6s are not naturally smooth but the 60-degree layout is due to evenly spaced firing pulses. there was a chevy 90-degree v6 (typically 4.3L) that was actually a small-block v8 with, literally, one cylinder deleted from each bank. GM has really done a lot to bring the little motor along to the 21st century.
rg

[yatesd]

Originally Posted by Ronster

as long as I'm at it I'd like to add some perspective regarding the engine in general. it's currently known as HFV6 for high-feature. this means it has 4 valves per cylinder, double overhead cams, variable valve timing, all aluminum construction, and other things high tech. it has really come a long way; it began as a chevy 2.8L ohv v6 near about 1980 or so, I think it first showed up in the citation (anyone remember that?). been used in camaros, s-10 pickups, cavaliers (the z-24) and its step-brother the cimarron, and heavy use in the lumina-type line (lumina, monte carlo [fwd] before 2000, olds achieva, etc), and beretta/corsica. the initial design advantage was the 60-degree bank separation which made it very compact and therefore light, and it naturally operates much smoother all compared to the typical 90-degree v6. buick had their 90-degree v6 (eventually ended up with turbos and intercooling in the grand national, beating corvettes at the time) but they had to either offset the crank journals or offset the spark timing to achieve some semblance of smoothness. more recently this 3800-series was used in the impala/monte-carlo line. 90-degree v6s are not naturally smooth but the 60-degree layout is due to evenly spaced firing pulses. there was a chevy 90-degree v6 (typically 4.3L) that was actually a small-block v8 with, literally, one cylinder deleted from each bank. GM has really done a lot to bring the little motor along to the 21st century.
rg

I could be wrong, but I don't think the "high feature" V6 is related to the original 2.8L. I originally owned the 2.8L in a 1989 Z24 Cavalier, and then had the 3.1L (bored out version) in a 1995 Lumina, Malibu, and Olds Cutlass (2002 I believe). I think it is still an option on some vehicles as a 3.5L (again heavily revised).

[Ronster]

Originally Posted by yatesd

I could be wrong, but I don't think the "high feature" V6 is related to the original 2.8L. I originally owned the 2.8L in a 1989 Z24 Cavalier, and then had the 3.1L (bored out version) in a 1995 Lumina, Malibu, and Olds Cutlass (2002 I believe). I think it is still an option on some vehicles as a 3.5L (again heavily revised).

I know Olds had a 3.5L v6 with multiple cams in the Intrigue (my daughter owned one). I thought it was related but it turns out that was derivative of the Northstar v8. I'm quite quite quite sure the current 3.6L hfv6 has lineage back to the chevy 60-deg. v6. there is a website dedicated to this design and its history. I have been wrong before (see the beginning of this paragraph) but I have looked this one up. on the internet. caveat.
I owned a 1988 beretta gt/z51 with the port injected (not sequential!) 2.8L and getrag 5-speed stick. also a fwd monte and an achieva with the sfi 3.1 ohv version. also note this motor went to 3.4L in the mid-90s lumina (euro sport and z34) with multiple cams.
judges?
rg

[proexpert]

Ron,

What is the carbon canister for?

Pro

[928S]

Originally Posted by yatesd

http://cars.about.com/od/thingsyoune...tinjection.htm

...Despite the additional power, EPA fuel economy estimates for the direct injection engine are 1 MPG higher in the city (18 MPG vs 17 MPG) and equal on the highway...

It seems that everyone wants to believe that DI is more fuel efficient, but contrary to what you've read here and elsewhere (I've seen the same misinformation on other websites and in print magazines) GM says...

http://www.cadillac.com/cadillacjsp/model/po_specification.jsp?model=cts&year=2009&section=Powertrain

EPA Estimated 18 MPG City and 26 MPG Highway (RWD 3.6L non-DI Auto model)
EPA Estimated 17 MPG City and 26 MPG Highway (RWD 3.6L DI Auto model)

Note: DI is estimated to have a 1 mpg advantage for AWD, Highway only.

[Ronster]

Originally Posted by 928S

It seems that everyone wants to believe that DI is more fuel efficient, but contrary to what you've read here and elsewhere (I've seen the same misinformation on other websites and in print magazines) GM says...
(gas mileage info)
....

DI is more fuel efficient than port injection. city gas mileage of the car is the end result of a lot of variables but the measure of efficiency is bsfc. brake specific fuel consumption means this engine makes more horsepower per unit of fuel used than the base model. so at the same load and rpm, the DI engine uses less fuel to do the same amount of work. check out the difference in final drive ratio between the different models and add the extra weight of awd, then you can compare the apple to the orange.
rg

[Ronster]

Originally Posted by proexpert

Ron,

What is the carbon canister for?

Pro

gas vapors from the tank (and elsewhere if applicable) are collected in the canister and condensed back to liquid form so they can be returned to the system, usually burned at start-up. it's a way of reducing hydrocarbon emissions to the atmosphere. in the bad old days the fuel system was simply vented to the outside air to allow expansion due to evaporation.
here's another difference with the hfv6, the fuel system providing pressure to the engine does not return excess gas to the tank like previous ones did. this keeps warmed fuel from flowing back into the tank which would generate more vapor so this change helps reduce emissions as well.
rg

[928S]

Ronster, apples to oranges is a good description of our messages. Like the EPA estimates and the quote to which I replied, I was talking about the CTS (w/DI engine vs. non-DI) as delivered from the factory. Sorry about the confusion.