Engine only MPG

I had to make several over 200 miles trips mostly on the interstate highway recently. With the gas price as high as it has been, I was more conscientious about the way I drive and was interested in finding out how the driving speed affects the overall average mpg for the trip.

Previous threads indicate that HV only mpg by PP is somewhere around 50mpg to 80mpg at highway speed. There are many variables and factors that would affect the results but we all agree that the faster you drive less efficient it gets.
Interstate Fuel Economy w/o EV Driving/Our Next Car? | PriusChat

For Gen3 (and maybe for Gen2?) there have been many technical discussions on the speed of the car and the rpm of the engine mapping to the efficiency. If IRRC, it was around 55-60mph which gives the best HV efficiency. I have not seen a similar technical discussion on the PP.

But for this discussion, without getting too technical, using only the gauge and display available to all PPs (and Gen4), I thought we can understand the relationship between speed and efficiency. When I was working with the data available on the screen, I realized that on PP and Gen4, the Drive Monitor displays the Average Speed, EV Driving Ratio, and corresponding Distance and Average fuel efficiency (MPG).

Here is an example of my recent trips with such data.


From such data, one can easily calculate the distance traveled on ICE only (not to be confused with HV only) and the amount of gas used for the engine. If I ignore the amount of the gas used purely for the warm-up cycle and maybe for the generation of electricity for the traction battery under HV mode, I can also calculate the ICE only mpg. That is the efficiency of the gasoline engine alone used for the traction of the vehicle.



I do not have many data points for my current 2021 PP for I do my in-town drivings mostly on EV mode only. But I had historical data from my 2017 and 2020 PPs, giving me enough data points to plot the ICE only MPG vs Average Speed for the trips.

What is interesting is that it shows roughly a direct relationship with a linear positive slope. That is, the faster the average driving speed is the better the ICE-only MPG. This seems to be counterintuitive. I always thought that the slower I drive the better fuel efficiency I can get.

Note that the overall average MPG in HV mode follows the inverse relationship to the speed such that MPG improves as the speed decreases.

So, can someone explain why this is happening? And with this knowledge, can one further improve the overall fuel efficiency while driving a PP (or Gen4) mostly on HV mode using the engine and gasoline?

 

Thanks. Your explanation is probably as good as what I can know without doing more technical testing on the engine efficiency. But just for the devil's advocate, if your explanation is correct, then driving a PP above 70mph would make the ICE only MPG to drop compared to say 40-60mph. My data points are mostly for my commuting drives for 20-40 miles at an average speed of 30-40mph and highway drives for 100+ miles at an average speed of 55-65mph. The highway trips do include above 70mph, but the average including off-highway or near the city region tends to drop down that number 10 to 20 mph. Next time I am on the highway, I may try recording just from the highway section where I can keep 70+mph. If you are correct, that data point should drop the ICE only MPG lower than ~45mpg as shown in the graph above.

 

Your observation is very similar to mine. What made me wonder about the speed on the highway and trying to keep good efficiency was that on the interstate where the speed limit is 70mph, it was very difficult to allow the EV to take over if I keep the speed at or above 70mph. The engine would be running all the time. Here is what happened when I did that on my recent trip. Most of ~274 miles were on interstate. The car was kept at or above 70mph most of the time. With an average speed of 61mph, the EV ratio was only 19%. and the overall mpg was 54.6mpg on the dash. This is a very typical number I get from PP for this trip if I keep the speed high. This did not use any portion with EV mode.


For exactly the same trip a week earlier with an exception that I had to make a short stop at the beginning of the trip, so the total mile is ~15miies shorter than the above trip. I have kept the car speed as low as possible without causing traffic problems (the minimum speed is 45mph). In order to keep the car coast and keep in EV longer, I had to do more "accelerating-and-gliding". The speed was never above 65mph but never let it drop below 45mph. It was mostly between 50-60mph. The average speed was 53mph and I achieved a 51% EV ratio and overall mpg was 72.0mpg on the dash. This is a very good mpg for the trip. Probably one of the best for me.


By comparing high-speed vs low-speed interstate trips, the former had ICE-only 44.226mpg and the latter with 35.28mpg. The overall MPG was better with lower speed of course, but the number on the ICE-only MPG was opposite of what I was expecting.

 

Can you explain what is the MPG ratio and the ICE MPG ratio you calculated?

Agreed. Your explanation makes perfect sense why the higher speed engine "appears" to be running more efficiently than the slower speed engine. From the data displayed on the dash alone, I have no way to document the amount of energy used for the charging. But it may be interesting to see the data from Hybrid Assistant on the highway speed trips. I don't know for sure if HA had the charging rate and engine rpm on its data, but IIRC, there was something like that. I have compared HA data before, but they were all for my shorter commuting trips of ~20miles one way. At that speed range of 35-55mph and short distance, there were not many variations. I think highway trip over 200 miles 55mph vs 75mph would show a bigger difference if such data are captured.

Yes, I do keep the old-fashioned empty to full tank method mpg calculation for each fill-up. But for a PP, especially now I don't drive every day for my work, the gas fill-up is far between. Without making long-distance out-of-town trips, a full tank can last for months for well over 1000 miles, even though most of my shorter in-town trips are a bit longer than the PP's EV range. Thus for each trip I make, the Drive Monitor data is a very good way to make a quick comparison from a trip to a trip.

On HV only drive for the same route and similar speed of the trip, the EV ratio seems to be the most important value influencing the overall MPG. The higher the EV ratio the longer the car was propelled using electrons rather than gasoline, thus resulting in a better MPG. For a non-stop drive, the Average speed is also a very important indicator of efficiency, but I have not done enough trips of the same route and distance with varying speeds.

For my example of ~260miles trips, one with a 61mph average speed could reach the destination ~38min quicker than driving the same distance at a 53mph average speed. But the slower speed of travel saved ~1.15 gal of gasoline. If time is not of the essence, then just slowing down will save the gas = $$$... and may save the earth if everyone joins.

 

I can see the "worse in the winter" part, but I don't understand why the full traction battery will increase gas consumption compared to the empty traction battery? I usually drive my PP on full HV mode in the winter morning commute with a full traction battery charge. The traction battery charge is used in my afternoon commute back home on EV mode. AFAIK, the MPG for the HV mode drive to work does not change much with full battery charge or no battery charge.

 

Oh, I see you are talking about using EV mode on the short drive mixed with HV mode. Yeah, then the mpg displayed is totally meaningless. But I am talking about MPG on the engine alone. No EV mode here. I often drive my car fully charged but on HV mode. The traction battery tends to drop 2-3% in HV mode due to the initial use of the traction battery. But this is the same if I had the traction battery empty. I get roughly the same MPG for my morning commute of ~18 miles all on HV mode with or without a traction battery charge. For that comparison, the dash indicated MPG is useful for comparison purposes even if it is not as accurate as we wish it to be.

 

Nop. I don't know how Pip displays this information. But on PP, there is no record of how many miles the car run on EV mode or how much kWh of charge from the wall it took. It only shows EV ratio which include the EV portion of the HV mode, and miles/kWh EV efficiency.

I am not wanting to know the engine only fuel efficiency. I was just asking why the engine only fuel efficiency I calculated appears to contradict the general notion that the slower the better mpg. @jerrymildred already gave me a satisfactory explanation. I am OK with his version of the explanation.

The average speed as I presented is only for the trips where there was no stops. Average speed was very good indicator of instant speed at any given point on that trip. It is not used for the calculation, but the plot I generated is ICE only MPG vs average speed. That was the point of this discussion.

Your point is well taken. But for my examples posted in this discussion, the average speed indicated on my Drive Monitor for the trip (After start till reaching the destination) was all on the highway, non-stop, with no traffic jams. The average speed compared are very good indicator of how fast I was driving the car at any given point for that trip.

All of my data presented here is 100% HV-only drive, there was no EV mode driven miles. I already have a very good handle on HV only MPG on my PP. For overall HV only MPG, DTE after a fill-up gives good enough estimation.
See this thread: What is your DTE after fill-up? | PriusChat
I have compared every single fill-up for the HV-only drive and hand calculated the real mpg by empty to full tank method. And compared the real mpg to the DTE estimation. The result is surprisingly good. Less than 5% discrepancy.

For a more accurate HV-only mpg calculation, I have my spreadsheet with formulas set up that will give me an overall lifetime HV-only MPG at any given point calculated from the number of kWh used to charge the traction battery and known EV efficiency.

Currently, my 2021 PP is at 6766 miles on ODO, the EV efficiency is 4.9 miles/kWh and overall HV-only MPG is 58.30. The table below is a copy of my spreadsheet. It is dynamic such that every time I enter new data with miles driven, the number of kWh charged, and the amount of gas-filled, the numbers in the table change. Since I also track the price of gas and electricity, it gives me an overall $cost/mile for EV and HV portion and overall average.

 

It would be nice if PP displayed separate miles for EV mode and HV mode and amount of electricity (kWh) for EV mode and the amount of gas (gal) used for HV mode. But since it doesn't, I made the table and way to calculate the HV mode gas-only efficiency from known data. Yeah, you can keep the record of miles driven on EV and HV for each trip, I actually did that to confirm my formula is accurate. I also drove my PP all on gas without charging for a full tank for a number of times to confirm the calculation. I don't have to do that anymore. If what I want to know is just HV mode gas MPG, then my table will calculate that from the amount of gas I purchased to fill the tank and the amount of electricity (in kWh) I used to charge the traction battery. It is not that hard.

But again, that is not the topic of this thread. I know what my PP's HV-only mpg is and what affects its HV-only efficiency. My question was simply the ICE-only efficiency (not HV mode efficiency where the EV used electricity is generated) appeared to be more efficient with higher speed, which was puzzling to me. Although I have no way to confirm, Jerry's explanation is a reasonable reason for this apparent counterintuitive result of my calculation for the ICE-only efficiency.

 

You are confusing the mpg on HV mode and ICE only mpg which is what I was asking on this thread. There is no P&G. There is no motor or EV involved. I was simply calculating the amount of gasoline used for the distance traveled purely on engine only no motor, no battery, no self-charging.