Sweet spot

[sgtlethargic]

What's the "sweet spot" of a gas engine relative to mpg?

Let's say you use the Ecomodder calculator and estimate a certain car needs 40 hp to cruise at 60 mph. Theoretically, would a 40 hp engine get the best mpg? Or, is it where the torque and hp lines cross on a graph? The area under some curve? Wide open throttle?

The followup idea is to add a turbo for acceleration needs.

What do you think?

Thanks

[ChazInMT]

The sweet spot in general is 70% throttle, 2500 rpm. this seems to fall in the middle of the place on the BSFC curve where the engine is most efficient. The Brake Specific Fuel Consumption curve illustrates the engines fuel consumption in relationship to the power that it is creating. Lower throttle and you have pumping losses where the engine is wasting energy sucking air past the throttle plate, and higher throttles have the extra power coming from dumping too much fuel in for the power produced, lower RPM and you aren't producing torque efficiently, and higher RPM has the engine wasting energy on friction to maintain the higher rotational speed. Search BSFC and you'll learn more.

[arcosine]

40 hp is more like a truck at 60mph.

[user removed]

Originally Posted by ChazInMT

The sweet spot in general is 70% throttle, 2500 rpm. this seems to fall in the middle of the place on the BSFC curve where the engine is most efficient. The Brake Specific Fuel Consumption curve illustrates the engines fuel consumption in relationship to the power that it is creating. Lower throttle and you have pumping losses where the engine is wasting energy sucking air past the throttle plate, and higher throttles have the extra power coming from dumping too much fuel in for the power produced, lower RPM and you aren't producing torque efficiently, and higher RPM has the engine wasting energy on friction to maintain the higher rotational speed. Search BSFC and you'll learn more.

Maybe 70% load? Throttle position and load can vary dramatically, just consider going from flat ground to a significant grade without changing throttle position. Load could go to 100% while throttle position may stay at 25%.

I think 80% load at 1600 to 2500 RPM is a good general rule for max efficiency.

regards
Mech

[Joenavy85]

I accelerate at 30% throttle (at least that's what my UG says it's at) until the TC locks at which point I gradually lower the throttle down to ~22%, and then down to 20% once I'm at cruising speed. This keeps me around 75-85% load for a majority of the acceleration time, then gradually lowers it down to 45-55%, which is where I cruise.

[ChazInMT]

By throttle, I mean the throttle position at which the engine is creating 70% of its power....not necessarily 70% of the travel on the gas pedal range of motion. We all have driven many types of cars and unless your braindead, you've noticed on some, you mash the gas pedal a tiny bit and yer moving on out (GM seems most like this) but others, you have to practically put your foot through the floor to get moving (I think Ford). Not saying one is better than the other, just different.
70% Throttle is 70% load, you're either slowing down climbing a cliff, going steady state up a very steep hill, or as is usually the case, you're accelerating on flat ground. I'm not saying you're going to cruise at 70% throttle, this is just the place where the engine is most efficient. This is why Pulse & Glide works well. The throttle position is the same as load since a governor is not in operation on cars. And if you look at 80% of the BSFC curves, 2500 RPM is in the middle of the island. You infer 2050 RPM is the middle of the island by saying 1600-2500 RPM. For large displacement engines and diesels, 2050 RPM may well be in the middle of the sweet spot, but for most smaller displacement gas engines, it seems like 2500 is the number. Technically, load is always 100% for any given throttle position, it is all just a matter of what the vehicle is doing with the power provided.

[user removed]

Load is better determined by MAP or the reverse vacuum. It is independent of throttle position as I stated in my previous post, if you go from flat ground to an uphill grade then the load increases and vacuum drops, while MAP (the inverse of vacuum reading) increases.

regards
Mech

[ChazInMT]

Originally Posted by Old Mechanic

Load is better determined by MAP or the reverse vacuum. It is independent of throttle position as I stated in my previous post, if you go from flat ground to an uphill grade then the load increases and vacuum drops, while MAP (the inverse of vacuum reading) increases.

regards
Mech

We are talking Gasoline engines here, not diesel. Rules are a bit different for a diesel engine.

Explain here how this happens in more detail, how is MAP Independent of throttle position? Obviously one of us has a misunderstanding. I say for a given throttle position, you are asking the engine to give you that much torque at a given RPM. MAP is going to be dependent on Throttle Position and RPM of the engine right? Going from flat steady speed to hill and not changing throttle will drop RPM obviously, but the torque the engine puts out will be relatively the same till you drop in RPMs below say 1700.

I think we are arguing the same point in different terms. When we look at a BSFC curve, it really has "Torque" as the Y axis (up & down one) and "RPM" on the X axis (side to side) with "grams per hour" ( or some given weight or volume of fuel being consumed over some period of time).

Since I don't think any of us has a "Torque" sensor, we need to rely on something that closely approximates torque for a given RPM of the engine and this would be the Manifold Pressure, or air pressure in the area above the intake valves. Again, since the only 2 things really affecting Manifold Pressure on a gasoline are Throttle setting and RPM, I don't see where my statement of throttle setting or position is wrong. The throttle setting will directly impact manifold pressure, and these are essentially 2 sides of the same coin. Since most folks don't have a scan gauge, and are able to detect MAP the only real way to gauge how much torque you are producing is by experience and feel for where your gas pedal is when you are creating about 70% of the rated torque of your engine.

If there is something I'm missing here, please educate and inform. Explain how it is wrong, or different, or if there is some third factor I do not understand.

[Bill in Houston]

how many angels can fit on the peak of a bsfc chart?