A Thought About Relative MPG

[aerohead]

Dave,I don't know if this is germane to the issue,but back in the 1970s,B.Dayman Jr. of Jet Propulsion Lab published a SAE Paper on coastdowns conducted at Edward's AFB,and he claimed that driveline losses were so great with an automatic transmission car in neutral,that they had to add a special clutch to dis-engage the propeller shaft from the tranny to normalize effects with a standard transmission car.
Could still be valid,don't know.If true,this might explain some of the 'discepancy' between the Impala and the F-350.

[comptiger5000]

I don't know if that's still true. While an automatic with the TC unlocked is pretty inefficient, they don't coast badly.

Both in neutral and in gear, at lower speeds (25 - 30 mph) where aero drag isn't an issue, my Jeep will coast practically forever. Then again, at those speeds, the thing actually coasts as well, or slightly better, in gear up to about 25 - 28 mph, as the engine is turning at, or just above, idle, and not dragging the Jeep down at all. Basically, that means that it's more efficient to coast in gear than to NICE-ON coast at low speeds, as the engine is using about the same idle fuel (as indicated by the instant mpg display), and still providing some motive power, rather than wasting it away in neutral.

[Big Dave]

I have often held that the torque converter is the tool of OPEC.

[darcane]

Even though both have 3.08 gears, your diesel still has higher overall gearing. The gearing in the tranny is different and you have taller tires on the truck.

Anywho, another datapoint for you: My truck is a 4.8L V8 Auto, 2wd with 4.10 rear end. On good behavior (no hypermiling tho) I'm a hair under 20mpg. Weighs in a 5400lbs with me and a full tank of gas (according to the scales at the dump). I'd love to have a manual instead, but I couldn't track one down. They were like hens teeth apparantly in '03 (and no longer offered at all).

[jkp1187]

Originally Posted by tasdrouille

There are already. Hyundai's new Sonata has a 2.4 liter GDI engine for example.

The 3.6L Cadillac CTS/STS engine is also DI, IIRC.

[Big Dave]

Do these DI gas engines have throttles?

[cfg83]

Big Dave -

Originally Posted by Big Dave

...

With a diesel and a stick, this is a 33-35 MPG car, easy.

Problem is that is not a clunker. It is pristine.

Ay, that's the rub. If you converted the Impala to manual, would you be able to "reverse it" to OEM if you so chose, or would there be irreparable harm, aka an extra hole for the clutch pedal or something? Would the carputer get upset?

CarloSW2

[Arragonis]

Originally Posted by Big Dave

One wonders when we will see DI gas engines in the US.

Will they be reliable?

We've had Mitsubishi GDi engines for a while but they were designed for the super clean fuel available in the Japanese home market, so some had carbon issues over relatively small miles - some people relearning the art of carbon removal :

Originally Posted by Honest John

GDI system of 1.8s prone to failure at around 60,000 miles due to carbon build up if cars are run on 95Ron petrol rather than the 97 - 99 Ron Super the engine is designed for. Same goes for Volvo S40/V40s with same engine.

I have seen evidence of Petrol engines getting better but nothing which is close to matching Diesel for overall efficiency, and I haven't see any evidence of Diesels being restricted in terms of torque/hp and efficiency.

In 2000 state of the art in Europe was 90hp / 150 lb/ft 45mpg, 100mph. Now its 160hp / 290 lb/ft, 55 mpg and 142 mph - both from 2 litres, Peugeot and BMW.

[tasdrouille]

Originally Posted by Big Dave

Do these DI gas engines have throttles?

Pretty much only the injection changed, they still need to control the AFR. You can think of it somewhat as an IDI vs DI diesel.

[darcane]

Originally Posted by Big Dave

Do these DI gas engines have throttles?

Yes.

An Otto cycle engine has to have something restricting airflow, as that is the only way to ensure a homogenous air/fuel mixture. The Diesel cycle does not have this requirement and can take in un-throttled air.

If you can figure out how to keep gasoline from detonating when the mixture isn't homogenous, then you can get rid of the throttle.