QUESTION: I recently purchased a 2010 Subaru Forester with the PZEV engine. It makes an awful clatter upon start-up after sitting overnight.
The salesman warned me about this noise before I even bought the car, explaining that it is normal. Is it? It is very disconcerting in a new car.
ANSWER: That "rattle" you hear is indeed normal for this particular engine.
It is estimated that 70 to 80 per cent of all hydrocarbons that escape out the tailpipe of a vehicle occur during the first seconds after a cold start - until the catalytic converter gets up to the temperature (300-350 Celsius) where it goes to work. As exhaust emissions have been sharply reduced, evaporative emissions such as these have become the toughest to tackle.
The catalytic converter used to be placed in the exhaust stream somewhere under the floor of the car. As these cold-start emissions became more critical, the "cat" has been placed as close to the exhaust port as possible, right up against the engine. The very rigid PZEV (Partial Zero Emission Vehicle) standards require evaporative emissions to be all but eliminated.
Subaru meets that standard by ensuring its PZEV models pair up with the special filler nozzle used in California gas stations, to eliminate any hydrocarbons that may sneak out during refilling - and by retarding the timing and taking other measures during the first few seconds to "light off" the catalyst more quickly than normal.
The noise you are hearing after a cold start is a result of those measures as the engine control unit (ECU) retards the timing slightly for a brief period to kick the catalytic converter into action without resorting to the extra-rich mixture normally associated with cold starts. Remember the choke?
QUESTION: I know you've addressed this one before, but my wife and I are having an argument about whether to get a high-mileage small car or hybrid.
It seems to me you have to drive a hybrid a long time to recapture the added purchase costs and that you would be just as far ahead financially to buy a diesel or other car with great mileage numbers.
What is your view?
ANSWER: The answer to your question depends on where and how much do you drive.
If you drive primarily in dense urban traffic with lots of stops and starts, the hybrid will be the way to go compared to a gas engine, with less of an edge over a diesel.
The big advantage to hybrids is that they are designed to shut the internal combustion engine off when the vehicle remains at a stop for more than a second or two with the brake applied - and restart instantly when you lift your foot off the brake in preparation for accelerating.
By shutting the engine down, it burns no fuel and has zero emissions. Taxi fleets are catching on to the benefits - in their case probably less a concern for the environment and more for reduced consumption and fuel costs.
The rate at which you recoup the added cost of a hybrid is directly related to mileage covered. Again, taxis operators see advantages.
On the other hand, if the majority of your driving is on the highway, a thrifty internal combustion engine may be the answer.
They normally get almost the same fuel mileage as a hybrid on the open road and don't have the upfront costs.
Just to complicate things though, if you plan to keep the vehicle for more than the normal two- to four-year span, and run high mileage, you may be faced with the cost of replacing the battery pack of the hybrid.
Diesel engines are rugged, built to withstand huge mileage and are at their best in the city, where they have the edge because of their low-end torque, meaning they use less fuel in stop-and-go situations.
But the cost of diesel fuel varies greatly across the country and this has to be taken into account, as does the occasional difficulty of finding a station that sells diesel.
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