[Vwdiesel] More on diesel efficiency

[hwy9fergs at comcast.net]

Svend said-
Ferguson ?   shouted down ?   --count on Svend not shouting.

Thanks for bearing with me.  I was hoping to make some sense out of this mess.
I wish I could take more time to devote to this stuff, but the reality lies elsewhere. I myself even get a little bored with this technical pontificating. Anyway, a quick note on hot starts in turbine engines.  The compressor section of the turbine engine is on the same shaft as the turbine section.  The hot exhaust flowing past the turbine is what drives the turbine- which drives the compressor, which supplies the air to the combustion chambers, which supply the driving force for the turbines which---------keeps the cycle going.  Once the engine has spooled up to normal operating speeds, the compressor puts out an abundance of air for combustion, as well as the air that is critical to be routed as a boundary layer to protect the combustion liners from the (extremely hot) flame. Without the air cooling the turbine engines could not survive. .  But during the starts, a starting motor of some sort is used to get the turbine turning to a certain point, but through most of the start, it's a fine line that gets danced around, adding enough fuel to keep increasing RPM, without putting too much in for the amount of air being supplied. When too much fuel is supplied in relation to the amount of air supplied by the compressor during a start, then you  have the "hot start" problems. Older engines with worn, less efficient compressor and turbine blades are more prone to this.  Huge flames coming out the tailpipe, melted turbine blades, etc.  Anyway, enough on that.
The other thing I wanted to mention on the diesel efficiency is interesting, in the way it differs from gas engines.  Specific fuel consumption on a gas engine is much more linear with respect to load.  If you look at a chart with the load and fuel use, the line would be more or less a straight line up at an angle.  For instance, a gas Jetta at 50 mph might get 35 mpg, and at 80 mph drop to 30 mpg.  With the diesel engines, the chart for specific fuel consumption has a line that stays relatively low down on the fuel consumption throughout the majority of the load range, until you get up near the full capacity of the engine, at which point the line starts to go just about straight up.  Looking at the way our mileage differs in the real world bears this out.  A 1.6 diesel (IDI) Jetta traveling at 50 MPH will get in the neighborhood of 45 mpg or better, but you take that same car up nearer the limit of it's capablity, say 80 mph, and you might more likely be looking at 35 or 36 mpg.  Just a little food for thought on your next long road trip.  Peace, Doug Ferguson