Vitesco Technologies, the powertrain business unit of Continental, is taking a less-traveled path toward paving the way for more affordable hybrid electric vehicles. The task is challenging due to the fact that a hybrid has two power sources: an internal combustion engine and an electric motor, complete with power electronics and a battery. These additional systems can raise the cost of an all-electric-capable high-voltage hybrid to a level that prohibits significant mass market penetration.
To address the problem, Vitesco developed a compact automatic transmission with an integrated high-voltage electric motor on the output side. The Vitesco concept reassigns a number of functions previously performed by the transmission (Fig. 1). This allows its new DHT transmission to use the electric motor to go from a standing start in first or second gear. Reverse is also done using the electric motor, while synchronization—equalizing speed with that of the next gear to be engaged—is performed by a starter-alternator that provides rapid starting of the internal combustion engine. In operation, the DHT reduces the number of mechanical components in the transmission, which saves space, weight, and cost.
“So far, it has not been possible to tap the full potential of plug-in hybrids and full hybrids for reducing CO2 emissions because the expensive powertrain of these vehicles puts them out of reach for many customers,” said Stephan Rebhan, Head of Technology & Innovation at Vitesco Technologies. “We have identified further potential here which our DHT technology for cost-effective PHEVs is designed to leverage.”
The DHT transmission has only four mechanical gears and has no mechanical synchromesh systems, auxiliary hydraulics, or a start clutch. Fully synchromesh transmissions became universal by the 1980s. Many modern manual-transmission vehicles, especially sports cars, now offer six- or seven-speed transmissions.
The DHT employs a simple dog clutch design; this is a type of clutch that couples two rotating shafts. The two parts of the clutch are designed such that one will push the other, causing both to rotate at the same speed and not slip.
Beyond the first two electric gears, there are four mechanical drive gears in the DHT. Vitesco engineers claim the combined system offers the driver “the same standard of comfortable driving and shifting” as a conventional six-speed transmission.
Using a low-cost port-injection gasoline engine and all-electric-capable drive, Vitesco expects DHT technology will make possible affordable vehicles that can perform a wide range of daily trips in all-electric mode, with zero local emissions (Fig. 2). The DHT for PHEVs is designed for speeds of up to 120 km/h in all-electric mode, and up to 160 km/h in hybrid mode.
Vitesco Technologies envisions advances in powertrain electrification as paving the way for genuinely affordable hybrid vehicles. “To ensure compliance with future EU CO2 emissions limits, it will be particularly important to get a grip on costs, which currently act as a barrier to the wider market success of hybrid electric powertrains,” concludes Vitesco’s Rebhan.