[Vwdiesel] Powerplant Trades

[Allan Reese]

There are a couple of things that can be added to Val's comments:

1.  Aircooled, at least those with reasonably high specific horsepower 
tend to be less fuel efficient because excess fuel is used to cool the 
engine.  This applies to both diesel and Otto cycle engines.  (The 
aircooled Deutz diesels I've seen shot visible exhaust smoke 50-feet 
into the air.)  That's why aircraft engines are run at "full rich" when 
using maximum takeoff power and then leaned out when the power is 
reduced.  Fuel is an expensive coolant.  On the plane I flew there was 
nearly a 30% reduction in fuel consumption when the engine was leaned out.

2.  Everything else being equal, a high compression engine (diesel) is 
more efficient than a low compression engine.  That's one of the main 
reasons why big truck engines are always diesels.  At partial throttle, 
diesel engines remain efficient whereas gasoline engines become less 
efficient.  I don't know for sure, but think this is because at partial 
throttle the effective compression ratio of gasoline engines is reduced 
because of the reduction in intake air pressure caused by the throttle.  
Maybe someone knows if this is true.

3.  Turbine engines are somewhat efficient at full throttle but they 
burn almost as much fuel at idle as they do at full power.  The pilots 
of twin engine turbine-powered aircraft often shut down one engine when 
taxiing to conserve fuel even though starting a turbine engine is very 
hard on it.   The ideal turbine-powered ship would have multiple 
turbines so that all are used at maximum speed and and as less power is 
needed some of the turbines are shut down rather than throttling all of 
them back.  This is hard to do mechanically, but relatively easy with a 
turbine-electric drive.

4.   There is no "best" engine.  It depends on what is important for the 
application--weight, specific fuel consumption, long life, etc.  I like 
my diesel engines but with today's relatively low-cost fuel the extra 
cost of a diesel as compared to a gasoline car can't be justified on the 
basis of fuel savings unless one drives a considerable amount per year.  
I got started with diesels in the early 70's when gasoline engines were 
burdened with emissions-reduction gagets that hardly let the engine 
run.  With today's catalytic converters, sophisticated ignition and fuel 
injection systems they run very well and are also much more efficient 
than they used to be.  Thirty years ago who would have thought that a 
full-sized car with a 300 hp gasoline engine could get 20 mph?  My '58 
VW bug with about a tenth that horsepower didn't get much better mileage 
than that.

5.  This is a bit off topic, but it is interesting to compare the 
hybrids, Toyota Prius in particular, and a diesel automobile.  The 
gasoline hybrids do very well in town compared to a gasoline non-hybrid 
but not as well when compared to a diesel.  On the highway a hybrid 
isn't much more fuel efficient than a non-hybrid because because the 
battery isn't used very much.  It is also interesting to note that on 
the Prius only about a third of the breaking is regenerative, i.e. 
putting energy back into the battery.  My guess is that this is because 
hard braking requires much more "horsepower" than the battery can 
absorb.  So gentle braking with a hybrid is far more fuel efficient than 
hard braking.  Of course what is really needed is a diesel-powered 
hybrid!  Anyone doing a conversion??

Allan Reese
areese at cascadelink.com



Val Christian wrote:

>Aircooled vs. water jacketed...the trades are simple.
>
>Aircooled: less weight, lower manufacturing complexity, looser engine 
>tolernaces, noisier operation, shorter life, less stable engine temps.
>
>Work OK for airplanes, chainsaws, and low milage cars.  (Didn't Saab
>have a 3 cyl aircooled engine?)
>
>Waterjacketed: weigh more, increased manufacturing complexity,
>tighter tolerances, better engine temp maintenance, quieter
>operation, longer life.
>
>
>Then there is the trade space for Otto Cycle and Diesel Cycle Engines.
>
>They fall into a couple of categories:
>
>-manufacturing complexity (higher for diesel, given the current state of the
>industry)
>
>-maintenance complexity (lower for diesel when averaged over the life of
>a properly maintained power plant)
>
>-wear limits (graceful degradation with the Otto, which tends to start
>more easily than a similar condition diesel)
>
>-power characteristics (torque, RPM trades, plus duty cycles, startup
>times, engine cycles, etc.)
>
>
>Well, our discussion could go on forever.  But there is the OTHER
>compression engine.  The turbine.  I don't hear of too many people 
>running these.  Yet, they are used in some powerplants, including
>some train engines (traditionally diesel in recent years (bar electric))
>If I could just get my hands on a nice turbine, with, say 40 hp.
>It would probably outperform a 48/52 hp vwdiesel IDI in auto applications.
>At least for highway drivability.  Remember, max torque at lowest RPM.
>Who needs a gearbox (save reduction gears)?  No more complex at starting
>than a diesel, although a hotstart has greater consequences.  The real 
>thrill is the noise you leave behind.  Then again, a 48 hp vwdiesel
>IDI without exhaust is a thrilling car...
>
>Val
>
>ps: I bet we can burn veggie oil in a turbine.  
>_______________________________________________
>Vwdiesel mailing list
>Vwdiesel at vwfans.com
>http://www.audifans.com/mailman/listinfo/vwdiesel
>
>
>  
>