Diesel mileage

[rh77]

Alright, I took this from the Volkswagen TDI club website -- I was an active member there when I was close to buying a Golf Diesel.

(I stand corrected about the detonation stuff)

"Air is drawn through a conventional air filter and into the so-called MAF, or mass-air-flow meter. A hot wire (older models) or hot film (later models) is contained inside this sensor and is maintained at a constant temperature. The electrical current which is required to maintain this temperature is an indication of how much airflow is passing over the sensing element. After the MAF, a hose connection comes from the valve cover where crankcase fumes are drawn into the intake air. From there, the intake air is drawn into the compressor of the turbocharger...where it is compressed, but the compression process also increases the temperature of the air. The hot compressed air passes through a small heat exchanger known as the intercooler. When heat is removed, the density of the air increases, thus increasing the amount (by mass) of air which is drawn into the engine. The objective is to make the air going into the engine cylinders as dense as possible (pressurized and cooled) to allow maximum power output. From the intercooler, the pressurized and cooled air goes to the intake manifold where it is mixed with a proportion of exhaust from the EGR (exhaust gas recirculation) system for emission control purposes. (The EGR system is connected on the high-pressure side of both the exhaust and intake systems.) This mixture then goes into the engine cylinders. Unlike with a gasoline engine, there is no throttle in a diesel engine. Power output is governed using the fuel supply only. The lean air/fuel mixture is one of the reasons that the diesel engine is so efficient, and the lack of intake restriction due to absence of a throttle reduces pumping losses, and that is another reason that a diesel engine is more efficient when running under part load."

OK, so hotter air would probably reduce power by reducing fuel pumpage -- that's my guess, and FE would increase. It's worth a try to bypass the intercooler and do an experiment (depending on how hard it is, and if you can keep the air clean -- post-filter mods need to be clean and well-made, especially with the high temperature and pressure of post-turbo operations).

I recommend the FAQ page from where I got this info -- very informative. Bear in mind that you have a "Common Rail" instead of "Direct-Injection" Diesel.

RH77

[GasSavers_DaX]

Originally Posted by rh77

Unlike with a gasoline engine, there is no throttle in a diesel engine. Power output is governed using the fuel supply only.

Holy cow...I never knew that!

[Sludgy]

Just got back from a trip to turkey hunting trip to Potter Mountain north of Pittsfield MA. I got 18.5 mpg in the F350, doing nothing differently than the trip to Monadnock....... 18.5 sucks, I suppose it's better than the 17 to NH.

[philmcneal]

can you bump start on a diesel? can it be abused even if the diesel has turbo?

[rh77]

There's a lot more torque and compression to bump and the turbo doesn't like to be immediately shut-down -- oil tends to sizzle on the hot bearings when they stop immediately and will kill the turbine over time. To get more than 100K miles out of a turbo, you need to idle it for at least 30-seconds if not a minute, based on oil temp. I had a timer on the EVO that would calculate the time based on oil temp and RPM -- you just turned off the car, locked it and walk away -- it would shut down automatically after the preset time. A waste of gas, but definitely a turbo saver.

My guess is, most late-model F-350 Diesels are automatics -- I assume yours is Sludgy? That would render bump start useless, but with the compression, Neutral coasting might help.

RH77

[Sludgy]

My diesel is an automatic. Push starting is impossible.

Cooking the turbo bearings doesn't usually happen unless you run the truck realy hard (like pulling a large trailer) and then shut down immediately. I'm pretty light on the pedal and run empty most of the time, so I don't worry about it much.

It was next to impossible to find a diesel with a manual transmission "on the lot". I did find one at a dealer, but it didn't have A/C or a locking diferential, both of which I want. I suppose I could have ordered one from the factory, but I needed the truck right away for an off road vacation in northern Maine.

[philmcneal]

i heard diesels are much more efficent in idling in N as opposed to a petro. So maybe abusing N alone could be worth it to getting good mileage?

[rh77]

Originally Posted by philmcneal

i heard diesels are much more efficent in idling in N as opposed to a petro. So maybe abusing N alone could be worth it to getting good mileage?

That makes sense -- the torque converter has to be huge to handle, well, all the extra torque. N would relieve the engine to have to spin that heavy connection.

[Sludgy]

According to the Scangage, my diesel uses 0.4 gph in N, when warm. In gear it's 0.6. It's much worse in cold weather, idling in gear at 1.2 or more.

I thought this was just due to high oil viscosity at low temps, but I just learned that Ford installed a valve in the exhaust pipe to choke the engine and make it work harder! This makes the engine warm up quicker.

My mileage drops off about 2 mpg in winter. Another reason for me to detest this Ford P.O.S.

[rh77]

Originally Posted by Sludgy

According to the Scangage, my diesel uses 0.4 gph in N, when warm. In gear it's 0.6. It's much worse in cold weather, idling in gear at 1.2 or more.

I thought this was just due to high oil viscosity at low temps, but I just learned that Ford installed a valve in the exhaust pipe to choke the engine and make it work harder! This makes the engine warm up quicker.

My mileage drops off about 2 mpg in winter. Another reason for me to detest this Ford P.O.S.

Yeah, the old non-turbo Diesels I experienced took forever to warm-up -- the valve was probably an EPA mandated change to get things to less-polluting mode. As for the Winter mileage do you have a special blend of fuel to reduce emissions like the gasoline? Otherwise, you may need to warm the incoming air (in any season -- too high though, and you could get some pre-ignitions problems, I've read - depending on the flash point of Diesel fuel. I'd seriously look into bypassing the intercooler.