“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Saturday, August 24, 2013

The (probably) last post on regenerative braking

I've posted a couple of times on regenerative braking in my CT200h. This will, I expect, be the last. In the previous post I estimated that regenerative braking on a trip saved me about 5.9% of the gasoline I'd have used without it. I decided that a better test would be a full tank, so I monitored all of the regenerated watt hours for my most recent tank. Since it's kind of a pain in the rear, I'm not going to keep it up.

Calculating in a more efficient way than the very detailed way in the previous post, the results are as follows:

  • The measured economy by miles divided by gallons at fill-up: 50.60
  • The calculated economy without regenerative braking: 47.55
  • Gallons per 100 miles: 1.976
  • Gallons per 100 miles without regenerative braking: 2.103
  • Per cent fuel savings: 6.04%
Not much different, so I think that it's safe to say that regenerative braking saves about 6% of the fuel I'd otherwise use.

I'm a bit surprised that the number is that low. In this post I discussed some of the factors that make hybrids so much more fuel efficient than their non-hybrid cousins and the regenerative braking was one of the factors I considered most important.

There is no non-hybrid CT with which to compare the fuel economy. I went to the DOE fuel economy site for the Camry (the four cylinder version)  and for the Camry hybrid. Using the combined highway and city estimates for each (28 m.p.g. and 41 m.p.g. respectively) it looks like the hybrid, per the government's test protocol, will use about 31.7% less fuel over any distance. It's reasonable to infer that, while the regenerative braking is a significant fuel saver, other factors (operating more frequently on more efficient areas of the engine map, capturing energy while coasting, automatic engine shut-off where appropriate, etc.) are at least as important.


Ed Davies said...

But haven't you been practising economic driving for quite a while? Perhaps a more typical driver would use the brakes more and hence get more from regenerative braking.

Rob Ryan said...

Yes, I've been "hypermiling" for about eight years. I use most techniques with the exception of "pulse and glide" even though I have the hybrid now, for which that technique is most beneficial. It's more effort than I'm willing to commit to fuel efficiency.

It could very well be that the typical driver would use the brakes considerably more and get more benefit, but I doubt I can find such a driver and get him/her to log the data I'd need.

I suspect that more benefit could be gained by having a larger battery, since I do see that the indicator shows a full charge. Surprisingly, the energy monitor shows charge going to the battery while braking or coasting even though the battery is shown as having a full charge.

I also wish that the monitoring system would show energy recovered during coasting. Oh well.

Arezoo said...

I volunteer to log the data after I get a new car.

Rob Ryan said...

Arezoo, I'll definitely be happy to take you up on that offer!