[Vwdiesel] Turbo vs. Non-turbo [was My TURBO test (more scientific)]

Tyler "Casioqv" Backman casioqv at usermail.com
Wed Apr 9 02:45:43 EDT 2003

> Accelerating quicker does not save energy. Period.

It does if you are accelerating quicker on energy that would otherwise
be wasted. Then you spend less time using the energy that is normally
avaliable. Basically what I was saying is that a more efficent engine
(turbocharged) can accelerate faster than a less efficent engine, and
still use less fuel. Of course, if you accelerated slower with the more
efficent engine, you would use even less fuel.

>>On my Volvo Turbo Diesel the engine produces about 7psi of boost on
>>level ground at 3000rpm during normal 75mph cruising for a long trip
>>(what I mostly use the vehicle for).
> What is max boost for this car, still ten?  Seven is higher than a Jetta
> needs, but it is lighter than a volvo I expect.

Actually, I am not entirely sure of the 7psi, or sure that my boost
gauge is calibrated correctly, as I am using a aftermarket faceplate
that calibrates the stock, non-calibrated gauge, which is not made to
very tight tolerences. I also might have been climbing a very slight
grade (I will check again, I just got the faceplate installed). Maximum
boost does seem to be about 10psi. Weight is not a major factor when
cruising on level ground at 75mph, and I suspect that most of the engine
power is used to combat wind resistance. Volvo 7 series are actually
much lighter than you would think considering the size, safety, and
chassis rigidity. I have not weighed my 760 turbo diesel, but my
gasoline volvo 740T is 2800 pounds, lighter than my friends TDI Jetta
wagon. Also, the engine is bigger (2.4 liters), and more powerful (about
110hp) which would likely make up for any increase in vehicle weight. It
seems to have about the same power curve as a 1.6TD. I am sure your
jetta gets better fuel economy, I get 36.5mpg with the turbo, and 30mpg
with the non-turbo, and hope to raise to 40mpg with some modifications.

> And at 75mph, I would expect to see some smoke behind your na version too,
> no?  I expect that your na is built to cruise in the 55-60mph range, no? ...
> just guessing.   Any engine operating near or at peak power will use more
> fuel than at "cruising speed" (a factor of power and gearing, resistance
> etc).  In percentage of available power, your na would be nearer the top,
> and not at it's most efficient level, where the turbo is farther from it's
> max power, and more in the cruising range. Just where you are operating in
> the available power range of an engine will effect the economy quite a bit.

Yes, the NA cruises at about 55mph, while the turbo likes to cruise at
75mph (would cruise higher if it had higher gears). At 75mph I would see
quite a bit of smoke on the NA, and would never cruise (or really ever
drive) it that fast. I plan to replace that motor with a turbocharged
one soon, because of the many advantages that we have discussed. I am
still experimenting with both engines to determine what is actually the
optimal cruise rpm (for best fuel economy), but the turbo does seem to
be at about 3000rpm, as was suggested to me.

>  If you go back far enough in diesel technology, yes, this would be the
> case.  HUGE turbos, that were sized to keep the rpm down, because metal and
> support technology had not progressed to the point where they could spin a
> compressor at 100000rpm without exploding into a collection of shiny bits.
> The sequence of events in a diesel is totally different than your gas cars.
> Stop comparing them and it will be easier, the combustion process is
> different.  This was what we started off disagreeing about... The scenario
> is, you give it part throttle under load with the intent of going faster.
> Your fueling instantly increases, as the governor speed limit has been moved
> via your moving your right foot. You have more fuel injected as the governor
> is calling for it, up to the fueling limit as set by aneroid and limit
> screw.  You now have instantly more exhaust gas volume and heat, the turbo
> spins up almost instantly (almost instantly in my volks, takes about two
> seconds in the semi with 3406 cat, about one second in electronically
> controlled cummins in my one tractor).  Your engine rpm's have not increased
> in any significant amount at all yet maybe twenty rpm, but the turbo is
> going like the lawn mower from hell. Engine rpms are less strongly linked to
> turbo rpms in engines  that run non-stoichiometric ratios because the energy
> in the exhaust products is much more variable.  That was what I was trying
> to get across in my original reply. In a gas job, the energy in your exhaust
> is a relative constant, just more or less of it by volume, because the fuel
> air ratio is supposed to be a constant factor.   You do not need to have
> engine rpms increase to have a turbo wind out and produce full boost in a
> diesel, you just need to put more fuel in. This can happen at any operating
> rpm.  Well other than idle...

Yes, but there is still some tradeoff with turbo size, but clearly it is
less so with a diesel, and especially with modern turbochargers (such as
the variable geometry turbo on a TDI). Turbo lag is almost a non-issue
at normal operating rpms. At low rpms 10psi would put the compressor on
the stock turbocharger above it's surge line, and does not occur with a
big enough turbo. This is the reason that you can't inflate your car
tires with your turbocharger, it is designed to only produce certain
boost levels at sufficently high flow rates (requires higher rpms or
more engine displacement), this is why the surge line on a compressor
map often resembles a "Y=X" graph within it's normal operating range.

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