In the war on tailpipe emissions, engine tweaks such as variable valve timing and direct injection are only popguns. The catalytic converter is still the bazooka.
The cleanest engines built today would not stand a chance of meeting the EPA's proposed Tier 2 standard without catalytic converters in the exhaust system. Even in the cleanest engines, the device cuts smog-forming hydrocarbons by 30 times or more.
Buying better converters also is generally cheaper than redesigning cylinder combustion chambers and adding exotic valvetrains. That is why automakers are focusing on the catalyst as the springboard for meeting Tier 2.
'After-treatment is it,' says GM powertrain engineer Allen Cline. 'Without the catalyst, you're not even close.'
Cline admits there is work still to be done on the engines themselves. If the ideal combustion chamber is a '10,' he figures that the industry is at 8 or 9.
But meeting Tier 2 will take a new generation of catalysts that can begin working within seconds after the engine starts.
Federal tests are based on sampling tailpipe gases collected over an entire round of cold and hot-run cycles. The quicker the catalyst begins working, the cleaner the samples will be.
Current models take 60 to 90 seconds to come up to operating temperature, but new designs that electrically heat the catalyst, inject small amounts of fuel into it or position the catalyst close to the exhaust ports can warm up the unit. Increasing the surface area of the catalyzing monolith is another improvement, but durability and cost issues are still being addressed.
For the next-generation Northstar engine, General Motors has developed a catalyst using twin air pumps to fan the combustion of unburned fuel in the exhaust pipes.
During the first 40 seconds after starting, the engine computer sets a rich mixture and retards the spark timing. When this half-burned mixture leaves the exhaust port and hits the piped-in air, it stays ignited. Cline refers to it as an 'exothermic reaction.' The burning fuel heats the catalyst to operating temperatures within 10 seconds.
Says Cline: 'The tailpipe is very clean once it is warmed up.'