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Hi folks,

I have a question, mostly theoretical, about the advantage/disadvantage of using "full throttle" in a diesel engine

Firstly, let me say that I know that diesel engines aren't throttled the way gas engines are, even if the TDI has an intake "flap" that accomplishes roughly the same thing, but for different reasons.

Theoretically, a gas engine runs most efficiently at wide-open-throttle (WOT) because the engine doesn't have to suck air past a partially-closed throttle plate. Of course, you won't get better fuel economy by flooring it from stoplight to stoplight, but judicious use of full-throttle at low-mid rpm while accelerating to a cruising speed can be more efficient than forcing the engine to suck air past a throttle plate while accelerating slowly to speed.

This was pretty much my driving style (conditions permitting) in my previous 2000 Mazda Protege ES; let out the clutch sensibly, apply full throttle to about 3500 RPM (or more if needed/wanted), upshift, repeat. I got pretty good gas mileage, and arrived at cruising speed as briskly as 120 lb/ft of torque could deliver.

With 116 MORE lb/ft of torque than the Mazda, my 2011 TDI is hilariously fun to drive in the same manner. I'm getting EPA-estimate mileage, possibly because I DON'T do this nearly as often. But I wonder: is there some theoretical advantage to flooring it in a diesel? There's no throttle to restrict the intake when accelerating slowly, and the turbo (positive manifold pressure) complicates things anyway. So is it vastly more efficient to lug it up to speed slowly?
 

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So is it vastly more efficient to lug it up to speed slowly?
I don't think I'd describe my in-town style as "lugging", but I am fairly light on the pedal and typically upshift at no more than 2200-2500 RPM. Is this more efficient? I suspect so. I'm averaging 37-38 MPG in my in town driving, well above the EPA estimate.
 

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As you've said both engines have slightly different fuel feeds. Petrol engines need to control the amount of air drawn into the engine which is proportionally mixed with the petrol either with a carburettor or injection. So engine speed/power is proportional to throttle opening.

Diesel engines generally have an unrestricted air flow and the engine speed/power is produced with increased amount of fuel injected. Some of the older diesels had a throttle valve so it would produce a vacuum needed for the brake servo. Other older diesels had a throttle valve because they had a pneumatic governor. Modern diesels use an exhauster (vacuum pump) to produce the vucuum for the brake servo.

When either engine is tested for power on a dyno they are always run at full throttle the engine speed is controlled by the load applied. The best specific fuel consumption thats the least amount of fuel to create the maximum power for both engines is the maximum torque rpm. Mine produces 258 lbs.ft @ 1750-2500rpm so driving anywhere in that range will give the best fuel con and torque. Diesels have a wider torque range today.
 
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