Is there a source for optimum engine rpm

[kane66]

I was wondering if anyone knows of a resource that provides information about different engines and there most efficient rpm range. I've found the information for a 1.6 NA diesel 1800 rpm's 265g/kwh, and the 1.9 TDI 700 rpms 204g/kwh and it got me wondering if there is a resource for all engines. If not maybe we could start a tread with different engines and there optimum efficiency. Just a thought.

[Concrete]

not meaning too be sarcastic but... 0 rpm is best for FE

I think you will find that most hypermilers minimize rpm
Engine off coasting being the extreme

I have an automatic and have limited control over RPM
so I use 30% throttle to optimum MPH - which for me is ~40 mph

don't lose sight of total installed efficiency - not just engine efficiency
you need to know how the engine - transmission - aero and rolling resistance play together

best way to know... is spend time with a scangauge

[cfg83]

kane66 -

If you can read BSFC charts, then this could be a resource :

http://ecomodder.com/forum/showthrea...-got-1466.html

CarloSW2

[meemooer]

i have thought for a while that optimum efficiency is right around your peak torque, but 3000+ rpms seems a bit excessive the way most newer cars are. So if you can find a dyno sheet of a factory car, i would guess it would be right where it starts climbing faster in torque, like between 1500-2500 for example

[kane66]

Concrete... unfortunately not all of us have the option to install a scan gauge on there car, unless you know of a way to install one one my 1985 1.6l vw diesel. (no ecm) So knowing what rpm your car is the most efficient is very necessary. For the best hypermiling drivers would try and stay in that sweet spot, or re gear there cars to hit that sweet spot at the speed the most often drive. For my car it is 1800 rpms. If I could set my car up to cruise at 60mph at 1800 rpms I would be getting the best fuel economy possible for that speed.

The BSFC charts is actually close to what I was talking about. What I'm used to seeing is g/kwh up the side and rpms along the bottom and a line graph showing how much fuel is consumed at each rpm. But I'm sure I could get that information out of the BSFC's if I learn how to read them. Thanks a lot for the suggestions.

[Concrete]

kane66,

sorry about that
I don't think we are far apart - confused on the question i guess
cfg83 & meemooer are right - best engine efficiency is way up in the middle to high RPMs
best vehicle efficiency is way low (like you said 1800 rpm)

What are you targeting with this data?

[Peter7307]

Kane66,
There are many factors determining the optimum FE for a vehicle as you already know.
One is the BSFC. Have a look at the Autospeed website and type in Brake Specific Fuel Consumption into the search box. This will give you a detailed explanation of how to read and understand those curves.

As a rule of thumb, most engines achieve their best fuel economy at an RPM corresponding to a piston speed of 5 to 6 m/s (16.4 to 19.8 ft/s). Piston speed (ft/s)= 2*stroke(inches)*rpm/720.

Piston speed = 2 x Stroke in inches x rpm / 720

As stated this is a rule of thumb only and specific cases may well alter the final numbers.

Pete.

[MechEngVT]

Originally Posted by Peter7307

As a rule of thumb, most engines achieve their best fuel economy at an RPM corresponding to a piston speed of 5 to 6 m/s (16.4 to 19.8 ft/s). Piston speed (ft/s)= 2*stroke(inches)*rpm/720.

Piston speed = 2 x Stroke in inches x rpm / 720

Where did you hear this rule of thumb? I had heard 20-25 ft/s but had heard other sources open it a bit more on the lower end.

You should be specific that the formula as entered gives piston speed in ft/s when you noted the rule of thumb values for ft/s as a side note. Nobody would be able to drive if they tried to work this formula to achieve an output of 5-6.

I find it easier to calculate piston speed in terms of feet per minute where the target range corresponding to 20-25 ft/s is 1200-1500 ft/min and the formula is

Piston speed= Stroke in inches x RPM /6

I personally find my best mileage when I center the engine's operating range around about 20 ft/s, shifting at 25 and letting it come down to about 16 in the next gear. Steady-state highway is shifted because my aerodynamics are pisspoor but I still find the best balance of fuel economy and time economy at 20 ft/s (75 mph). I can achieve better fuel economy at slower steady speeds, but I have to drop at least 10 mph to see any significant increases at which point driveability makes it harder to maintain FE due to constant downshifting (5spd trans).

[kane66]

I guess what I would like is the ability compare engines ability to convert fuel into usable power. As everyone knows not all engines are created equal. And even though you might have two similar engines there relative efficiency may be completely different. And if I was to build a car specifically designed for fuel economy I would want the most efficient power plant. I'm currently planning a project to put a VW 1.6TD engine in a 91 Honda civic HB. I always assumed that it would get better fuel economy than the 1.9 TDI. But as I stated above the 1.9 is actually %25 better at converting fuel into power. So if I had the money the 1.9 would have been a much better choice. Unfortunately I didn't know that at the time, and I already have the parts so 1.6 it is. So I guess that's were I'm coming from and why the request for the information. If anyone wants to see what I was talking about check out this PDF it has the fuel usage chart on page 8.

http://www.haywood-sullivan.com/vana...i_specs_en.pdf

I guess a pdf brochure of each engine like this would be nice, but like they say if wishes were wings, pigs would fly.

[some_other_dave]

You're ignoring one other factor: Load. Engines tend to be more efficient at making power when under larger loads, generally around 70-80% or so.

Check the BSFC charts, they have the info you need. The "islands" on them are the amount of fuel used per unit power made, while the X and Y axes correspond to RPM and load.

Note that when you are in a steady-state cruise, your load will be quite small, and will depend on quite a few things (rolling resistance, aero drag, temperature, yadda yadda yadda). So the BSFC charts (and the fuel/power vs. RPM charts) are much less useful at that point... But for acceleration, they're kinda nice to see!

-soD