VE vs MPG
07-19-2004, 11:58 AM
#1
Cap'n Limekitten says arr
Thread Starter
Join Date: Jul 2003
Location: Falls Church, VA
Posts: 491
Likes: 0
Received 0 Likes
on
0 Posts
VE vs MPG
I'm sure everyone is aware that increasing the speed of any engine increase fuel consumption. However, my question kind of deals with a special scenario, and may be a plain stupid question, but ...
If the VE of an engine increases over a small RPM range, say, from 90 to 100% over the 4000-4200 rpm range, will the fuel efficiency of that engine go UP instead of down?
As an example scenario, imagine you get 25mpg in car X by travelling 300 miles on the highway at 70 mph. If you increase the speed to 72-73 mph, can you get the same or better mpg, while still shaving some time off the trip?
If this is nonsense, I apologize, there was a lot of time to ponder such things on yesterday's 300 mile trip
If the VE of an engine increases over a small RPM range, say, from 90 to 100% over the 4000-4200 rpm range, will the fuel efficiency of that engine go UP instead of down?
As an example scenario, imagine you get 25mpg in car X by travelling 300 miles on the highway at 70 mph. If you increase the speed to 72-73 mph, can you get the same or better mpg, while still shaving some time off the trip?
If this is nonsense, I apologize, there was a lot of time to ponder such things on yesterday's 300 mile trip
07-19-2004, 12:44 PM
#2
Registered User
Join Date: Apr 2004
Location: Canyon Lake, TX
Posts: 186
Likes: 0
Received 0 Likes
on
0 Posts
Is your VE is Volumetric Efficiency? In any case the specific fuel consumption is what is important. That is the fuel used per power generated, regardless of size. It generally is a graph shaped like a "U" and a engine is poor at low and high speeds. There are many things that affect this and one is VE.
Some, especially in avaiation, have used the term BMEP for the break mean effective average pressure required on the piston to produce the power that the engine generates, in PSI. It is good as it takes all other considerations out, including size and type of engine, in comparing engine efficiency. Aviation used it for the old large piston engines as a way for the flight engineer to adjust the propeller pitch for best efficiency.
But in your case the required power goes up with the switch from 4000 to 4200 as the load goes up from both air and road drag. So even if the VE and SFC are better for the engine in your example the actual fuel required will go up and your mileage will go down.
There are some other considerations that can come into play. One is for rolling hills. You may be able to better maintain a average MPG by going fast enough on the down hill to go up the next up hill without dropping down in rpm to a poorer SFC point or requiring a heavy foot or downshifting to a higher engine speed and resulting higher fuel usage.
Some, especially in avaiation, have used the term BMEP for the break mean effective average pressure required on the piston to produce the power that the engine generates, in PSI. It is good as it takes all other considerations out, including size and type of engine, in comparing engine efficiency. Aviation used it for the old large piston engines as a way for the flight engineer to adjust the propeller pitch for best efficiency.
But in your case the required power goes up with the switch from 4000 to 4200 as the load goes up from both air and road drag. So even if the VE and SFC are better for the engine in your example the actual fuel required will go up and your mileage will go down.
There are some other considerations that can come into play. One is for rolling hills. You may be able to better maintain a average MPG by going fast enough on the down hill to go up the next up hill without dropping down in rpm to a poorer SFC point or requiring a heavy foot or downshifting to a higher engine speed and resulting higher fuel usage.
Thread
Thread Starter
Forum
Replies
Last Post