Hi,
This article got me thinking about the 30 mph problem:
Power can co-exist with fuel efficiency
Quote:
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Powertrain improvements cut fuel consumption enormously. BMW has two new engines and an all-new automatic transmission. The new gasoline-powered, turbocharged 3.0-litre in-line six not only uses direct fuel injection and a twin-scroll turbo (one scroll is fed by cylinders 1, 2 and 3, while the second scroll gets its exhaust gas from the remaining three cylinders), it also uses Valvetronic.
The latter eliminates the throttle and the restriction it represents. In this case, it's the valvetrain that controls the engine's speed and output.
The use of direct injection and turbocharging reduces fuel consumption by 21%. Throw in the 8% savings supplied by Valvetronic and this engine is 29% more fuel efficient than a conventional 3.0L six. With 300 horsepower and 295 pound-feet of torque (at a low 1,200 rpm), it promises to deliver the snap of a V8 with the economy of a six.
The second engine is a 3.0L straight-six turbodiesel. With 300 hp and a monster 442 lb-ft of torque on tap, it promises to be a wild ride. The key to its power production is the fourth-generation direct-injection system, new fuel injectors and an elaborate turbocharging system. In a nutshell, the system uses two different turbochargers. The smaller variable-vane turbo spins quickly up to speed, which banishes lag. At 1,500 rpm, the second larger turbo comes on line. The two then work to deliver the desired throttle response. At 3,200 rpm, the smaller turbo is taken off-line to prevent it from becoming an exhaust restriction. Factor in the high (for a diesel) 5,500-rpm recline and you have an engine that outmuscles BMW's former 4.0L V8 diesel while dropping fuel consumption by 23 per cent.
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The 30 mph problem is it takes about 5 hp (3.7 kW) to sustain a modern sedan. But this is about the same amount of energy needed by the engine alone to turn over at low power settings. So about 10 hp is needed of which about 50% is used to turn over the engine and 50% used to keep the vehicle moving.
In contrast, a Ford or Toyota hybrid will run the engine for a short period of time to generate about 20 hp with 5 hp the engine overhead, 5 hp to the wheels, and 10 hp (7.45 kW) being stored in the traction battery. Roughly 25% of the energy runs the engine and 75% is used for traction power either when the engine is running or later from the traction battery.
Any vehicle that requires the engine to run at 30 mph, diesels, gas, or hybrids with little or no electric vehicle mode, will pay about 50% of their tank just turning over the engine. In contrast, a hybrid with significant electric vehicle mode gets a much improved efficiency, nearly double that of the other. Now granted, these numbers are approximate but they also match the specifications of the Toyota, Ford, and Honda hybrids and starter motor power needed by diesels and ordinary gas engine vehicles.
So I can admire BMW's efforts to improve vehicle, transmission and engine efficiency. But until they understand the 30 mph challenge, their results will remain as disappointing in the future as they are today.
Bob Wilson