The future-and what we'd like to see come down the road next
A major hurdle when trying to maximize engine efficiency is planning how to reduce the parasitic drag created by a belt-driven engine's accessories like the alternator, air conditioning and power steering. Audi engineers set out to tackle the problem on the A4 TDI Concept e, developing a secondary voltage source independent from the engine to power the electronic rear brakes, electromechanical steering, and a host of other electronic goodies. They accomplished this by using an energy-recuperation system that converts kinetic energy during deceleration into usable electrical energy which is stored for use during acceleration, effectively reducing drag, increasing performance, reducing emissions and increasing fuel economy. There's also a start/stop function that shuts the engine off at a standstill. Put it in neutral and the system disconnects the air conditioning's compressor from its drive whenever possible. All this results in a vehicle that produces 120 hp and 213 lb-ft of torque, zero-to-62 times of 11.4 seconds, and 58.95 mpg.

With the early success of the 1 Series, we'd hope that BMW would add the 123d to the list of vehicles it'll be bringing to the U.S. market. Powered by 2.0-liter, four-cylinder engine, the 123d produces 204 hp at just 4000 rpm and 295 lb-ft of torque at 2000 rpm. It's enough to propel the vehicle from zero to 62 mph in seven seconds and maintain a 45 mpg efficiency rating. Behind the 123d's impressive numbers is the world's first four-cylinder diesel engine to use Variable Twin Turbo technology, where sequential turbos are employed to optimize power and torque across the rpm range. Each turbo has been sized to do a specific job. The smaller, low-inertia unit spools instantly to effectively create immediate throttle response. As demand increases, the larger unit takes over to ensure there's continuous boost as engine revolutions increase.

The line dividing diesel and petrol performance has become so blurred that it's impossible to simply look at a vehicles' performance numbers and determine if it runs on petrol or diesel. A perfect example is Mercedes' C-Class. Based on the vehicle's zero-to-62 time of seven seconds, you might assume it had a V6. Then you notice the vehicle is able to net a combined efficiency rating of 45 mpg. It has the power of a six-cylinder with the fuel economy of a four-cylinder. That's better than a Prius... plus it's prettier and handles better, too. The C250 CDI is also somewhat unique when it comes to its high-performance diesel powerplant. Instead of a turbocharger, its 2.2-liter inline four utilizes a dual-stage supercharger to produce 204 hp and 368 lb-ft of torque.

While a highly efficient engine design is important, the Volvo DRIVe package for the C30, S40 and V50 demonstrates that if you start with the engine and tailor the vehicle itself and drivetrain around it, you end up with increased fuel economy and reduced emissions. To reduce drag, the DRIVe package consists of a series of aerodynamic enhancements, specially designed wheels, reduced vehicle ride height, and a front end designed to optimize airflow into the engine bay. It is further enhanced with a next-generation Michelin tire with decreased rolling resistance, tweaked engine management, and optimized cooling and power steering systems. To round out the package, the transmission has been re-geared and filled with a new low-friction gear oil. All this adds up to a 4 to 7 mpg increase on a C30/S40/V50 equipped with a 1.6-liter diesel engine. In addition to the mileage increase, zero-to-62 times were reduced from 11.4 to 10.8 seconds and emissions reduced by 8.5 percent. If you're not into economy-sized engines, Volvo also offers a 2.4-liter, five-cylinder turbo diesel engine that can propel a S40 to 62 mph in just 7.5 seconds.

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