Thursday, December 30, 2004


Will Diesels Spell The End For Parallel Hybrid Drivetrains?

It seems that in the past few months, there has been an increasing amount of information coming out about the development of diesel-electric hybrids by certain OEMs. This indeed is a logical development, as the primary purpose of hybrid technology is to conserve energy (whether they do so or not is the topic of another post), and without a doubt, a properly-utilized diesel offers an advantage over spark-ignition internal combustion engines through two primary means - an increased compression (and thus, expansion) ratio, and the elimination of pumping losses across the throttle plate (reduced by some extent by lean-burn, cylinder-deactivation, and fully-variable valve lift technologies, but never quite eliminated).

Many future hybrid designs depend on the use of a CVT transmission to keep the engine working at a point where the engine can develop the required power (a combination of the power required by the dynamics of the vehicle itself plus the desired amount of energy to be stored in the reserve mechanism). Sure, it's possible to use a stepped-ratio transmission in this application, but hybrids require optimization of, well, damn near every aspect of the powertrain in order to provide a usable improvement in economy. CVTs also provide a wide ratio spread in a small package, which is additionally important in a hybrid due to the added packaging volume that the electric drive brings with it. One can see that a CVT is simply the right transmission for the job.

But CVTs bring with them one major drawback, and that is a lack of torque capacity. 300 Newton-meters of torque capacity is considered by many to be the current benchmark, and at that, they are not suitable for use even with many of the smaller highly-turbocharged four-cylinder diesels, much less one of the larger diesel engines one might expect to find in future large-car and light-truck hybrid applications. Quite simply, the diesel's high cylinder pressures and low operating speeds are going to result in a significant amount of torque in order to meet the average customer's requirements for average (and to a lesser extent in hybrids, instantanous) power output.

But the use of diesel-electric hybrid drivetrains in applications such as locomotives and off-highway trucks has been going on for 50+ years at this point, due to the need to generate a huge amount of torque at a standstill, and the inability to develop a multi-ratio transmission and clutch or torque converter that will accomplish this task efficiently. The solution? Simple - a series hybrid drivetrain, where in the engine is only connected to the drive wheels through the generation and energy-storage mechanisms.

This completely eliminates the need for any transmission, and indeed, taken to the logical extreme, eliminates the needs for a drivetrain if the electric motors are simply mounted at the wheels. Doing so brings about its own problems (an potential increase in unsprung weight, for one), but there would be obvious advantages to eliminating the mass and packaging volume requirements that come with traditional drivetrains - the elimination of distinct vehicle platforms and all the restrictions they bring to new designs would be a start. In all likelihood, a series system would be less complicated than a parallel system (remember, we've been building locomotives and dump trucks like this for a long time), and it probably brings additional efficiency to the table as well.

Of all the good reasons we have to build series hybrids, it just might end up that we go in that direction due to the lowly torque-handling capabilities of the CVT, a problem that's as old as the CVT itself (going right back to the days of Van Doorne). Or, hey, maybe someone will decide that something like hydrostatic drive is really the way to go and the parallel architecture will live on for another few model cycles, allowing the current OEMs to become even more firmly entrenched in their current thinking and manufacturing capabilities.

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