Toyota Rav 4 Prime Charge Mode

Over the last three months of owning this plug-in hybrid, while making the weekly drive between Boston and Maine, I have been taking data. The objective: to answer with some confidence the question of just how much less fuel efficient is Charge Mode vs the car's normal Hybrid Vehicle driving mode. For clarity, the Charge Mode on the vehicle uses the Internal Combustion Engine (ICE) to charge the Electric Vehicle (EV) battery. For comparison, Hybrid Vehicle (HV) mode uses a combination of EV battery power and ICE power to improve fuel efficiency.

It's well known that ICE's are more fuel efficient on highways, while EV's are much more efficient on local roads. Our route is a mix of the two. As I wrote about in a previous post, the trip from Boston to Maine includes 3 miles of stop-and-go driving, followed by about 90 miles of highway driving, followed by about 20 miles of rural roads. The question here is not whether it might be more efficient overall to drive in Charge Mode or HV mode. If that were the case, Toyota Hybrid technology's complicated switching between ICE and battery would be pretty pointless. Rather, the question I'm trying to answer is how the two modes compare specifically under mostly highway-driving conditions (where ICE is most efficient and EV least efficient).

Toyota representatives have spoken about the charge mode as a specialty mode with no use other than in Europe, where one may conceivably travel to a city with a restriction on gas-burning vehicles. Other Rav 4 owners have commented that of course the efficiency of using battery power generated by an ICE is less efficient than utilizing the battery directly as in HV mode, as the charging process is highly inefficient. But I think there's one advantage a human has by manually switching driving modes over the onboard computer: we know the route ahead. I thought, depending on how much less fuel-efficient Charge Mode actually is than HV driving mode, there may be a use case, where, if local driving on a trip totals more mileage than the battery capacity of the car, that charging the battery during a highway leg might make sense for gas mileage.

This is the first installment of my data, stretching from Summer through most of Fall, using the methodology outlined in that previous post. Big thanks to Micha for logging the mileage every time I drove. As the data is a bit noisy (but not actually as noisy as I expected), I chose to just present it directly, without a lot of data reduction, because sometimes metrics can obscure more than they illuminate.

Here's all the data in MPG presented in chronological order along the X axis. Red bar calculates the raw MPG numbers in each mode. Blue bar adds in the accumulated EV range, which is only relevant for Charge Mode, but presented for all modes for completion.

As the weather got cooler, our raw EV range reduced from a peak 54 miles to the current 49 miles. But neither HV mode nor Charge mode has (yet) suffered a loss of efficiency.

Reorganized based on driving mode, a comparison can be made. Same data, sorted so that Charge Mode data is on the left and HV Mode on the right. Comparing the red bars, we can see that driving in Charge Mode has a much lower raw fuel-efficiency than HV mode, about 30 MPG vs 40 MPG on average. However, comparing the blue bars, we see that the efficiency is more or less made up when the added EV range is accounted for. The particularly tall 50 MPG bar was due to an extended traffic jam really stretching the EV range that we had charged up on the drive. Though it is just a single data-point showing significantly better performance utilizing the Charge Mode, it is suggestive that when used thoughtfully, there may be some advantages.

Reordered based on driver, we can learn a bit more. Micha (last 6 sets of bars in the graph below) drives about 10 MPH faster than I do. I always fall behind Google Maps estimates, but, my driving style (first 11 sets) is much more conducive to saving fuel. On average, driving 65 MPH improves fuel efficiency about 5 MPG over driving 75. In Hybrid Mode, this is about 38 MPG vs 43 MPG. In Charge mode, this is about 28 MPG vs 32 MPG. You make up for it a bit because you charge up the battery more for the same distance driven, but it doesn't quite compensate for the fuel-efficiency loss due to that velocity-squared drag term.

Since I've streamlined the data-taking process, I generally look forward to each new data point. That's why I consider this the halfway point of this study. I'm not bored of it yet. We've driven a total of 2000 miles in this study so far, and are averaging about 900 miles to a tank of gas. We're expecting our first snow storm up in Maine next week, so I'll see whether there is something to be said about weather and road conditions.