Reviewing the Prius UK-Style
By Chris Ellis
Chris Ellis is the founder of PowerBeam, a UK-based automotive development firm. He regularly contributes to EV World, providing insight into automotive technology trends in Europe. He recently test drove a European-version of the ‘05 model Prius to assess its suitability for installation of his firm’s unique flywheel technology. Below is his perspective on the current world-standard in gasoline-electric hybrid vehicles.
I drove the car for over 400 miles over a variety of roads, including motorways with the cruise control set at 85 mph, and also up one of the hills used for more than a century to test English prototypes. I tried to adopt different driving styles, to establish which owners would be happy with their new purchase, and who wouldn't want one. All the mpg figures came from the onboard system, and have been converted to US gallons; 5 UK gallons get you 6 US gallons.
The highest average figure I saw was 65 mpg; I'm not claiming that as any kind of record, merely an indication that 'my' Prius was fully up to spec. The mpg figure that most impressed me was the 52 mpg I obtained between Minehead and Barnstable, across sometimes hilly terrain and always winding narrow roads, at the sort of speeds a typical sales rep might employ. If the car had been a conventional 'clean diesel' of similar cabin volume and weight, it might have done 40 to 45 mpg, or 35 to 40 on gasoline. On this route, I rarely exceeded 70 mph, below which speed the Prius is a pleasure to drive reasonably hard. If I tell you that the fuel consumption over the section up Porlock Hill averaged 12.2 mpg and this is included in the 53 mpg, you'll understand that the Prius engine can both dish it out and deliver what is promised on the tin, i.e. excellent fuel consumption at all legal UK and US speeds.
However, I did notice an annoying tendency for the engine rpm to fluctuate quite widely with only minor changes in accelerator position, rather as if the transmission was fitted with a torque converter from the 70's, before lock up clutches were in widespread use. The engine also seemed more eager to rev beyond 2,500 rpm than a conventional automatic. You will recall that the principal reason locking torque converters were introduced was to improve fuel consumption. I suspect Toyota may already have prototypes running around fitted with a 'MkIII HSD' which locks the transmission above a certain road speed, in the manner of a conventional overdrive. The design priorities for the original HSD were probably Japan first, California second and 'hard core' European drivers forty third. That may now change.
Some time ago, Mercedes identified in public the 'Achilles heel' of the current HSD, namely the losses that occur at higher speeds, with the generator needing to supply power to the motor via the controller even when the battery is effectively out of the picture. This has been one of the reasons why DaimlerChrysler has joined with General Motors to develop an alternative which is more efficient at freeway speeds. However, the fix we have identified to further improve HSD may well allow Toyota to match the DC/GM solution at all speeds. Mercedes (and/or GM) will need PowerBeam to pull decisively ahead. (A very short commercial - it's over now!) The maximum speed limit in the UK on motorways and dual carriageway roads is 70 mph, and most European countries have similar limits.
However, in a touching gesture of European solidarity, some drivers in France, Italy, Spain and the UK try to drive at German speeds whenever they can. In practice, this means a vehicle needs to be able to accelerate swiftly from 60 to 90 mph to begin to qualify for the 'outside lane club', where the drivers who aren't paying for their own fuel like to congregate. This is where Prius currently loses a big chunk of addressable market, because fuel taxation policies and the sheer cost of fuel are motivating both owners (often companies in the UK) and drivers to chose cars which operate efficiently at these speeds. For example, it is said that some 70% of all new S-Class Mercedes registered in the UK now have diesel engines. And the Prius, despite its efficient engine and relatively low drag coefficient, doesn't do that well at these speeds.
To take my twitchy right foot out of the loop, I set cruise control at an indicated 87 (a true 85?) on a clear section of the M5 motorway over the Somerset levels (as flat as you'll find this side of a salt lake), with and without air conditioning engaged. Over a five mile stretch, I got only 35 mpg with air conditioning on. This improved over the next five miles to 38 mpg with the AC off. Given that a US government test showed a 30% increase in fuel consumption over the federal cycle for a Mk I Prius, 3 mpg worse isn't so bad, but this is essentially a function of the much higher average traction power being deployed at a steady 85. The main reason for the Prius's relatively low fuel consumption at European cruising speeds must be lie within the transmission, and these high losses are confirmed by the (rare) need for the transmission to be water cooled.
EV Mode EV mode is apparently not available in the US, for reasons I suspect have something to do with product liability. As evidence, the following quotes are from the UK version of the Prius Owner's manual.
- 'In the "EV" drive mode your vehicle runs like an electric vehicle, only using the electric motor....'
- 'CAUTION The driver should pay full attention around the vehicle especially when it is driven by the electric motor (with the gasoline engine stopped). People in the immediate area might misjudge the hybrid vehicle movement based on the absence of the regular engine noise.' Not only in bold, but inside a black border!