Please try this simple thought-experiment about a little car trip:
Assume you have a gasoline-powered, non-hybrid automobile that gets an average 24 miles per gallon. (Metric values below for non-U.S. readers.)
- Walk out of your house in the morning, get in your car, and drive 4 miles to the gym. Stay there an hour.
- Then drive 4 miles to a shopping center. Stay there an hour.
- Then drive 4 miles home.
(Metric: Assume the three legs of the trip are 6.66 km each and your car uses 10L/100KM.)
Approximately how much fuel did you use for the entire round trip? Assume your travel has the same city/highway ratio used to compute the mileage rating.
Did you answer half a gallon (2 litres) ?
Probably you used at least 50% more than that or 3/4 gallon (3 litres). So your effective mileage for the trip is more like 18 mpg (15L/100KM) or worse.
Why? Because an engine is much less fuel-efficient before reaching its normal operating temperature.1 This means about the first 7 miles/11 km after a cold start. Each of the three legs in our example is more-or-less a cold start, although maybe by the time you get home it will be all warmed up.
(Even if you live in a hot climate, starting your motor after a few hours idle counts as a cold start.)
Pre-warming your engine in the driveway makes it worse. The gasoline you need to use for that is burning inefficiently while getting you zero mpg….
Unfortunately, most Americans car trips are short, and their fuel usage consequently is vastly higher than you would expect from the mpg ratings. According to U.S. government statistics, almost half of automobile trips are less than 7 miles.2
That’s where a PHEV (plug-in hybrid electric vehicle) comes to the rescue.
Maybe you’ve thought about buying a Tesla? Probably you haven’t considered buying a PHEV. They aren’t sexy!
PHEVs are typically dismissed by both industry and consumer journalists alike. Why? Because their relatively small battery packs typically provide only 20 miles or so of all-electric propulsion, before the gasoline engine needs to kick in. “Big deal”, or some other sarcastic comment, is the usual reaction.
Pictured: the author’s Ford C-Max Energi PHEV
But 20 miles of all-electric, for most drivers, is a huge win in energy efficiency. A typical PHEV owner can turn probably half of all gasoline-powered trips into all electric. (And electric motors don’t need to warm up!) So for a long trip, you burn gasoline and enjoy your car’s rated mpg, but for short trips, you avoid the cold-start mileage penalty. Many people can re-charge between trips, such as those who work at home, as so many do these days.
And unlike Tesla, no electrician needed: A regular household 15 or 20 amp circuit provides enough juice to fully recharge most PHEVs overnight.
This a huge win not just from an environmental point of view, but also for the pocketbook. For what you pay in gasoline to carry you for three short cold-start trips, you could get about 15 of those same trips, on electric power.
And a bonus: your gasoline engine lasts longer in relation to its hours of use (i.e. excluding hours on battery power). Most engine wear occurs just after a cold-start, when moving parts are not yet fully lubricated, because the oil is still cold and thick. If you can cut your number of cold starts in half, you may well double the operating time before that engine needs any serious repair work.
PHEVs are all the more attractive today because they are cheap, relatively speaking. Leaving aside the federal tax credit for new models, I’ve seen good bargains in the used marketplace. PHEV’s just aren’t sexy enough to merit the premium price that Tesla and other full electrics command.
Why doesn’t the automotive press discuss this? Probably because most reporters take shortcuts and recycle information from other writers and fed to them by the industry.
And American auto manufacturers and importers themselves don’t want to sell too many of these models. They aren’t the big money-makers. Their profits come from SUVs and pickup trucks with big conventional engines. Government fuel efficiency mandates 7 require them to produce a certain minimum number of environmentally friendly vehicles each year. And so the minimum number is what they turn out.
Of course society would be much better off if all those SUVs and pickup trucks were plug-in hybrids. But nowadays the auto companies are obsessed with trying to catch up to Tesla. The trouble is, the world can’t make all-electric cars the default, yet. We don’t mine enough lithium for all those batteries, and the engineers still have work to do on the technology with respect to range and recharging convenience.
Meanwhile, a PHEV is a nice cheap compromise.
You heard it here first. Even though you should have heard about a decade ago from all the parties mentioned above, when the first PHEVs came on the market.
- “Your fuel economy is worse when your engine is cold than when it is warmed up. So, several short trips taken from a cold start can use twice as much fuel as a longer, multipurpose trip covering the same distance.” U.S. Dept of Energy, Office of Energy Efficiency & Renewable Energy
- US. Dept. of Transportation, Bureau of Transportation Statistics, Average Annual PMT, VMT Person Trips and Trip Length by Trip Purpose. (See trips for “shopping, school/church, other” in 2017)
- Tax credit info is irrelevant for used cars; only the original purchaser can claim that.
- It should be discussed on this page but isn’t https://fueleconomy.gov/feg/phevtech.shtml
- See for example, Motor Trend, “Four Reasons to Go With a Plug-In Hybrid and Four Reasons Not To“, which fails to mention gasoline’s cold-start penalty for short trips.
- See Consumer Reports “How to Get the Best Fuel Economy Now“, which should suggest combining short trips into one longer one, but fails to.
- a.k.a. Corporate Average Fuel Economy (CAFE) regulations