2010 Lexus HS 250h
2010 Lexus HS 250h. Click image to enlarge

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By Haney Louka; photos by Paul Williams

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2010 Lexus HS 250h

Winnipeg, Manitoba – It was difficult at first to know for sure, and somewhat anticlimactic when we did confirm it: Lexus’ first dedicated hybrid model, the HS 250h, does not actually possess any new hybrid technology. That’s okay, though, because the car comes from a company that sits at the forefront of mass-production hybrid technology. So Lexus had a veritable smorgasbord from which to pick and choose the most appropriate technologies for installation in the new HS.

But first, the basics. The new HS drives on what is termed a “parallel-series” gasoline-electric hybrid powertrain. Such an arrangement is used on all Toyota and Lexus hybrids (along with Ford and Nissan) and is distinct from Honda’s system in that the car can drive on electric-only propulsion in a variety of low-speed and low-load conditions. Honda’s “mild” parallel hybrid system can now shut off the gasoline engine under deceleration and coasting conditions, but actual propulsion still requires the gasoline engine to operate. Both systems, however, have an automatic stop function that is activated when the vehicle comes to a rest.

Now back to the HS: the gas engine is the 2.4-litre Atkinson cycle unit from the current Camry Hybrid. The Atkinson cycle engine works on the principle that a lower exhaust temperature yields a more thermally efficient engine, i.e., less energy is lost as waste heat out the tailpipe. Lengthening the power, or expansion, stroke in an engine accomplishes just that because it lengthens the amount of time between combustion and the opening of the exhaust valves. But doing so also lengthens the compression stroke which effectively increases the engine’s compression ratio. Increase it too much and knock, or pre-ignition, will result. The Atkinson cycle overcomes this hurdle by delaying the closing of the intake valves as the compression stroke begins. This way, the compression ratio is lowered while still achieving a high expansion ratio.

2010 Lexus HS 250h
2010 Lexus HS 250h. Click image to enlarge

This increased efficiency comes at the expense of power production, since less air is compressed each cycle. So the Atkinson isn’t a great fit for conventionally-powered cars. But in a hybrid such as the HS 250h, where an electric motor provides a significant boost to the gas engine’s output, Lexus says it’s a perfect fit.

On its own, the gas engine produces 147 hp but is complemented by a 141-hp electric motor (plus a second electric motor that functions as both a starter and a generator for the battery pack). While I’m sure Lexus would have been happy to add those two numbers together for a final output figure, the reality is that the two power sources cannot simultaneously operate at peak output. So the real number is a somewhat less awe-inspiring 187 total system horsepower, same as for the Camry Hybrid.

The third major component in the hybrid powertrain is an electronically-controlled continuously-variable transmission, or CVT. The CVT is far from a new concept, and certainly not my preferred method of putting power to the pavement. But for this hybrid, the CVT is the brain of the whole operation. It is the programming of the CVT that allows the HS to be as frugal or spirited as possible by altering chosen drive ratios in any given situation.

While most of the decisions are left up to the computer that controls the CVT, Lexus does allow the driver some control over the car’s demeanour by way of their four-mode driving system first introduced on the 2010 Prius. In addition to Normal mode, there are Power, Eco, and EV modes from which drivers can choose.

Power mode is fun, and provides the HS with the kind of responsiveness I’d like to see every day, but it pretty much defeats the purpose of having a hybrid. It’s great to have this option, for times when a spirited drive or imminent highway pass is on the agenda, but the quick throttle response and elevated engine revs provided by the CVT go against minimizing fuel consumption.

Normal is middle-of-the-road, while Eco tells the CVT that its driver is looking for maximum efficiency. Not only does Eco mode retard the throttle response and keep engine revs down, but it also reduces the air conditioning performance to minimize load on the hybrid drive system.

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