Data fuel consumption: Hybrids vs. Diesel vs. Regular gas engine car

I really think their numbers for Toyota on the highway is off
A lot of my driving is at about 105-110 and there is a bit of hills
And It will usually be at 4.5l/100km

 

Thank you for the data:

The data looks reasonable:

Note that I used Euro style mileage combined with SAE style speed. This should equally confuse folks on both sides of the Atlantic but the clueful will understand the data. Each has only one factor to convert.

My estimate is based upon these assumptions:

• Otto cycle partial power efficiency << full power operation. This is seen in the Skoda efficiency fall off as the speeds decrease. This comes from the pistons having to work against a partially closed throttle plate that causes a partial vacuum.
• engine overhead ~= displacement. This is seen in the linear projection of the 1.6 L diesel and the 1.8 L Atkinson from the lowest two data points of both vehicles. The overhead comes from piston, valve, oil pump, water pump, crank shaft and rod overhead. Engine friction, energy loss, the overhead, is pretty much linear with rpm.

But the Toyota HSD has one more trick at constant speed that the current diesels can not automatically follow.

The Toyota HSD can cycle the ICE on and off at speeds under 46 mph. During the time the ICE is off, there is no engine drag. So the engine generates enough power to provide battery power so MG2 can use it to sustain the constant speed:

	Column 1	Column 2	Column 3	Column 4	Column 5
0	MPH	linear Toyota	linear Volvo	cycled Toyota	%
1	44	4.30	3.61	4.30	100%
2	37	3.91	3.31	3.76	88%
3	31	3.53	3.01	3.22	75%
4	25	3.15	2.71	2.69	63%
5	19	2.77	2.41	2.15	50%
6	12	2.38	2.11	1.61	38%
7	6	2.00	1.80	1.07	25%
8	0	1.62	1.50	0.54	13%

The HSD operation intersects the diesel at 25 mph. In contrast, the diesel has to pay the engine mechanical overhead constantly when it is running. To sustain a constant speed, the diesel still has to spin regardless of how little energy is needed for propulsion. It still has pay the engine mechanical overhead cost.

This is just a basic model as there are other effects that begin to predominate at lower speeds. For example, vehicle electrical overhead, generator-battery-motor efficiency, and loss of engine heat. The diesel is faced with minimum rpm challenges at lower speeds. Certainly a more complete and sophisticated model can be constructed but the expected, overall results should parallel this analysis down to ~10 mph.

We saw a similar result with the "Green Human", Portland-to-Portland, 8,000 mile, highway-speed, endurance drive two years ago. Using the 1.5L, Prius, 40.96 MPG, versus the 1.8L Jetta TDI, 41.4 MPG, the mileage difference was 0.44 MPG, less than 1%. In spite of their best efforts, the Jetta TDI, a compact car, delivered less usable volume than the Prius for the same mileage.

Notice I haven't touched the relative energy content of diesel versus gasoline nor fuel costs. Furthermore, the relative passenger and luggage volume of these Finnish vehicles is an important indicator of utility. Also, I haven't looked at NOx vs COx loads which the California Air Resource tests have shown the Jetta TDIs to be about 5x greater than the Atkinson Prius. The Prius HSD also allows operation at very low rpm. A more sophisticated model would include these additional factors.

Once again, thank you for the data. I believe it provides a basis to understand Prius performance versus the Volvo diesel.

Bob Wilson

ps. The European version of the EPA mileage tests:

http://www.carkeys.co.uk/road_tests/volvo/volvo_v50_16d_drive_se_with_startstop.aspx

http://www.carkeys.co.uk/road_tests/toyota/toyota_auris_hybrid_t_spirit.aspx

http://www.carkeys.co.uk/road_tests/toyota/toyota_prius_tspirit.aspx

 

Thanks to a PM, one aspect of this chart, low-speed mileage, needs more detail:


What happens is as the speed goes slower and slower, the fuel efficiency falls off due to:

• vehicle overhead - when the vehicle power consumed is ~500 W but the power needed to sustain a constant speed is much lower, ~200 W. For example, a human can push a car on a flat surface at 2-3 mph and we're typically rated at 1/4 hp, ~150-200 W. The vehicle overhead would exceed the human output and have no energy for propulsion at any fixed speed.
• engine overhead - for those vehicles that have to run the engine at any speed over 0 mph, the engine will consume fuel to overcome engine internal friction. This overhead energy loss can be several times that required to maintain a constant speed in the 1-10 mph range.

The original private message was right that at very low speeds, the fuel efficiency falls off again as seen in the gray line (MPG ~= k / liters_per_100_km):


Every vehicle has a maximum range speed, in the 1.5L Prius close to 18 mph. Go faster and range goes down. Go slower and the range goes down.

I had tried to suggest that we would need more data to make a more complete model. In particular, we need the fuel consumption at engine idle. There is a simple, engineering rule of thumb that can be used to calculate the maximum range speed:

• vehicle_overhead + velocity_load = ICE_idle_load

Bob Wilson