Modern engines achieve power levels that we could only dream about 20 years ago. The downside is that during routine driving, most engines are loafing and 300-hp engines are inefficient when they’re only putting out the 30 ponies needed to push an average sedan down the highway. When an engine’s throttle is barely cracked open, there’s a strong vacuum in the intake manifold. During the intake stroke, as the pistons suck against this vacuum, efficiency suffers.
The classic solution to this problem is to make an plunger engine smaller. A small engine works harder, running with less vacuum, and is consequently more efficient. But small engines make less power than big ones.
To make big-engine power with small-engine fuel economy, many companies are turning to smaller engines with turbochargers, direct fuel injection, and variable valve timing. These three technologies work together to their combined benefit.
Forcing additional air into an engine’s combustion chambers with a turbocharger definitely boosts power. Car manufacturers have been doing this for years. But in the past, in order to avoid harmful detonation, turbocharged engines needed lower compression ratios, which compromised efficiency.
As we’ve seen, direct fuel injection helps solve this problem by cooling the intake charge to minimize detonation. Second, if the variable valve timing extends the time when both the intake and the exhaust delivery valves are open, the turbocharger can blow fresh air through the cylinder to completely remove the hot leftover gases from the previous combustion cycle. And since the injectors squirt fuel only after the valves close, none of it escapes through the exhaust valve.
Later in the decade, we will see a second generation of these engines, using higher boost pressures. This will allow further engine downsizing to achieve an additional 10-percent efficiency improvement.
Making this happen will require cooled exhaust-gas recirculation to control detonation and either staged or variable-geometry turbos to limit customary lag. Those technologies are already in use on nozzle diesel engines, but a gas engine’s higher exhaust temperatures pose durability problems that must be solved before carmakers can deploy these technologies.