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bwilson4web · 11-27-2009, 08:58 AM

I was planning to write a separate thread about the drag and engine characteristics of the 1.5L NHW11 Prius. However, it makes sense to write it up as a kind of appendix to the Graham mini scanner guide.

Engine BSFC and Vehicle Drag

The following chart shows the Graham mini scanner recorded, Brake Specific Fuel Consumption:

Unlike traditional BSFC curves, this chart uses "bubbles" to indicate the measured, relative efficiency of the 1.5L engine in use ... the operational line. The small red dots are periods of highest engine efficiency. Then come the larger orange dots, yellow, light blue and finally, deep blue. They are plotted showing the ICE torque versus rpm, the operational line gathered from the vehicle operating in traffic. Finally, there are the small green engine HP marks, the measured ICE power output.

This chart uses the drag formula Ken@Japan found to generate the 'power required' line:

The red "HP" line is the dot product of the drag force over the distance per unit of time. It is the amount of power needed to sustain the vehicle at any given speed. The blue "MPG" line is the calculated MPG with a 31% efficient engine and no other vehicle energy costs. The gray "MPG with fixed" adds the measured, vehicle electrical load needed for the control computers, daylight running lights and brakes. To these curves I've added data points from mine and other benchmarks.

Practical Application

Operating the vehicle to keep the engine in the peak efficiency region will provide the best, overall vehicle performance:

1,200 to 2,600 rpm - although in practice, I prefer to keep a guard band so 2,400 rpm is my efficiency peak rpm.
1,300 to 1,650 rpm - high efficiency cruise power, when acceleration is not required.

Drag ultimately dictates how much power is needed at any given speed. So using the "red" HP required line, we can determine how much power is needed at any given speed. Then using the Graham mini scanner derived BSFC chart, we find the "knee(s) in the curve." We simply choose to travel at speeds that stay on the fuel efficient side of these knees.

Now in any given system, there will be subtle 'knees' that are not intuitively obvious. The BSFC chart shows 1,700 to 1,900 rpm suffers a loss of efficiency. This is in the 19-23 hp range and to the left of the knee in the drag based, "HP" curve. It is an area where the vehicle MPG seems 'flat' and not following the traditional V{2} curve. If the engine were more efficient, we would expect a higher MPG value at these lower speeds ... 60 mph is not significantly more efficient than 65 mph.

The more interesting gap is around 2,400 rpm where there is a drop off of red and orange dots. This corresponds directly to the 65-70 mph knee and as the speed increases, the required HP rapidly moves into less efficient engine regions. I treat 65 mph has my normal, cross country, cruise control speed.

One last chart of interest is climbing an 8% grade hill:

Climbing the hill at 55 mph turns out to correspond with an ICE ~2,400-4,150 rpm. Going up the hill faster pushes the engine into the really inefficient regions:

This is a fuel BTU test of different brands in the Huntsville Alabama area climbing an 8% grade hill at 55 mph held by the cruise control. I was surprised to find the vehicle control laws treated 4,100 rpm as a boundary. Neat Easter Egg revealed from the Graham mini scanner data.

Traditional BSFC Charts

The traditional BSFC chart is created by an engine in a dynanometer with a wide open throttle. The dynanometer manages the engine speed, not the throttle, and the dyno measures the generated power. In real world driving, the throttle controls engine power. In some cases, the water pump is not powered by the engine, a little known 'cheat.' In contrast, the Graham mini scannner, BSFC chart shows the operation in urban and hill climb traffic with partial throttle used as needed and includes the water pump overhead.

Similar, field BSFC charts can be produced IF the vehicle has a torque sensor. One approach is to mount sensors on the clutch plate "springs" but this not trivial. Another approach is to mount torsion sensor in a drive shaft or even the wheels but again, these are not trivial tasks. The NHW11 measures torque because MG1 provides the counter torque to the ICE in the power split device.

Bob Wilson

11-27-2009, 08:58 AM	#3 (permalink)
bwilson4web Engineering first Join Date: Mar 2009 Location: Huntsville, AL Posts: 843 17 i3-REx - '14 BMW i3-REx Last 3: 45.67 mpg (US) Blue Bob's - '19 Tesla Std Rng Plus Thanks: 94 Thanked 246 Times in 157 Posts	Appendix - NHW11 ICE and Vehicle Characteristics I was planning to write a separate thread about the drag and engine characteristics of the 1.5L NHW11 Prius. However, it makes sense to write it up as a kind of appendix to the Graham mini scanner guide. Engine BSFC and Vehicle Drag The following chart shows the Graham mini scanner recorded, Brake Specific Fuel Consumption: Unlike traditional BSFC curves, this chart uses "bubbles" to indicate the measured, relative efficiency of the 1.5L engine in use ... the operational line. The small red dots are periods of highest engine efficiency. Then come the larger orange dots, yellow, light blue and finally, deep blue. They are plotted showing the ICE torque versus rpm, the operational line gathered from the vehicle operating in traffic. Finally, there are the small green engine HP marks, the measured ICE power output. This chart uses the drag formula Ken@Japan found to generate the 'power required' line: The red "HP" line is the dot product of the drag force over the distance per unit of time. It is the amount of power needed to sustain the vehicle at any given speed. The blue "MPG" line is the calculated MPG with a 31% efficient engine and no other vehicle energy costs. The gray "MPG with fixed" adds the measured, vehicle electrical load needed for the control computers, daylight running lights and brakes. To these curves I've added data points from mine and other benchmarks. Practical Application Operating the vehicle to keep the engine in the peak efficiency region will provide the best, overall vehicle performance: 1,200 to 2,600 rpm - although in practice, I prefer to keep a guard band so 2,400 rpm is my efficiency peak rpm. 1,300 to 1,650 rpm - high efficiency cruise power, when acceleration is not required. Drag ultimately dictates how much power is needed at any given speed. So using the "red" HP required line, we can determine how much power is needed at any given speed. Then using the Graham mini scanner derived BSFC chart, we find the "knee(s) in the curve." We simply choose to travel at speeds that stay on the fuel efficient side of these knees. Now in any given system, there will be subtle 'knees' that are not intuitively obvious. The BSFC chart shows 1,700 to 1,900 rpm suffers a loss of efficiency. This is in the 19-23 hp range and to the left of the knee in the drag based, "HP" curve. It is an area where the vehicle MPG seems 'flat' and not following the traditional V{2} curve. If the engine were more efficient, we would expect a higher MPG value at these lower speeds ... 60 mph is not significantly more efficient than 65 mph. The more interesting gap is around 2,400 rpm where there is a drop off of red and orange dots. This corresponds directly to the 65-70 mph knee and as the speed increases, the required HP rapidly moves into less efficient engine regions. I treat 65 mph has my normal, cross country, cruise control speed. One last chart of interest is climbing an 8% grade hill: Climbing the hill at 55 mph turns out to correspond with an ICE ~2,400-4,150 rpm. Going up the hill faster pushes the engine into the really inefficient regions: This is a fuel BTU test of different brands in the Huntsville Alabama area climbing an 8% grade hill at 55 mph held by the cruise control. I was surprised to find the vehicle control laws treated 4,100 rpm as a boundary. Neat Easter Egg revealed from the Graham mini scanner data. Traditional BSFC Charts The traditional BSFC chart is created by an engine in a dynanometer with a wide open throttle. The dynanometer manages the engine speed, not the throttle, and the dyno measures the generated power. In real world driving, the throttle controls engine power. In some cases, the water pump is not powered by the engine, a little known 'cheat.' In contrast, the Graham mini scannner, BSFC chart shows the operation in urban and hill climb traffic with partial throttle used as needed and includes the water pump overhead. Similar, field BSFC charts can be produced IF the vehicle has a torque sensor. One approach is to mount sensors on the clutch plate "springs" but this not trivial. Another approach is to mount torsion sensor in a drive shaft or even the wheels but again, these are not trivial tasks. The NHW11 measures torque because MG1 provides the counter torque to the ICE in the power split device. Bob Wilson __________________ 2019 Tesla Model 3 Std. Range Plus - 215 mi EV 2017 BMW i3-REx - 106 mi EV, 88 mi mid-grade Retired engineer, Huntsville, AL Last edited by bwilson4web; 11-27-2009 at 10:24 PM..