More after the jump
The first thing Toyota did was fit the Prius with an Atkinson-cycle engine. The Atkinson cycle leaves the intake valve open for part of the compression stroke. While that seems counter to efficiency gains, it keeps cylinder pressures in check and allows longer burns, ensuring more complete combustion and extracts more work from the intake charge. The lower cylinder pressures reduce NOx emissions by keeping temperatures down. We'd be curious to see the 1NZ-FXE engine in the Prius fitted with Toyota's trick "dual-injection" system. The dual-injection system uses both direct gasoline injection, into the cylinder, and indirect injection, into the intake port. The reason for the seemingly redundant systems lies in optimizing swirl and atomization. Teaming the two methods together allows more optimal tuning across the RPM range, resulting in better efficiency and performance.
Sticking to the engine, there's an insulated canister fitted to the Prius that works like the vacuum flask we fill with fine coffee every morning (lest we have to stoop to store-bought swill). Warm engine coolant is held in the canister and then re-used to speed engine warmup. This is a really slick trick and would likely bump every engine's MPG rating a hair, as time spent warming up would be greatly reduced. Another thing to consider for colder climates would be gasoline-fired coolant heaters. These are pretty commonly fitted to vehicles in Scandinavian countries. They consume a fraction of the gasoline used by a running engine and are a far more efficient way to pre-warm your coolant. It'd also mean an end to our neighbor's seemingly endless idling at 4AM. WIth the coolant pre-warmed, the engine is able to get into it's most efficient mode of operation much faster.
An automatic transmission (or CVT like the Prius) is undoubtedly the way to go for emissions control. With fly-by-wire throttles, the conditions of shifts can be tightly controlled so that the engine management isn't chasing the driver's input like less integrated systems have to do. The throttle plate can be feathered, timing optimized, fuel delivery exactly metered, all to ensure a perfect shift as cleanly as possible. Team the transmission with a steep final-drive ratio, and program it to short-shift, keeping the throttle plate open wide, and pumping losses are brought down. We're not sure what the current state of throttle-less gasoline engines is, but the last time we checked, it was difficult to get them to meet emissions targets. While it would seem great to just run the thing as lean as possible, cleanliness suffers. With a lean mixture, there's not as much fuel to absorb heat in the cylinder, leading to excessive cylinder temperatures and very high combustion pressures. Those lead to increased NOx, which is one of the more difficult internal-combustion byproducts left to control; engineers have done a great job figuring out how to tame most of the others.
A low coefficient of drag can have a marked effect on MPG at elevated speeds. The Prius has a low .26 Cd. Lessening the wind resistance through body shape has to be the most maintenance free way of lowering consumption and thus emissions. Those savings last for the life of the car (provided it's repaired after any crack-ups). Tires that offer low rolling-resistance are also a proven way to reduce fuel consumption. The tradeoff is typically that ride and handling suffer, but we're not sure how true that is anymore. A lot of it comes down to tuning the car for its prospective audience, and efficiency-geeks don't typically feel the need to go whipping around corners (though you could do it, and be efficient at the same time!).
Lastly, weight reduction is a huge factor. Cars today are heavy. Using composites and aluminum where possible helps, and safety equipment isn't light, either. Not only that, vehicles are expected to come full-boat loaded. Power everything, nice stereos, HVAC that'll roast marshmallows in the winter and chill sodas in the summer, etc. The Prius really shines at delivering a remarkably normal driving experience for all its technogeekery. We're just curious how the Prius would do without the weight of the battery pack and electric motor, and a little more gumption from underhood (perhaps throw a supercharger on the engine, making it a Miller Cycle).
Let's make sure we're clear here - this is not Prius hating. We're just wondering out loud about possible solutions that would yield across-the-board efficiency gains for all the cars that are not blessed with Hybrid Synergy Drive. We can learn some new tricks from the hard work that the Prius team has undertaken, and we'd love to see Toyota lead the way applying bits and pieces across their entire line of vehicles.