Hi folks,
When looking at the efficiency of any system, the general rule of thumb is the product of all efficiencies is the total, tank-to-wheel efficiency. For example:
MECHANICAL
(35% ICE) * (95% 1st gear) * (95% 2nd gear) * (95% 3d gear) =
(35% ICE) * (86% three gears) ~= 30%
SERIES ELECTRICAL
(35% ICE) * (90% generator) * (90% converter) * (90% motor) =
(35% ICE) * (73% transfer) ~= 25%
However, this simple model ignore the operating range. The flaw is high ICE efficiency is available only in a narrow speed range. Partial power modes become much less efficient and this is where the mechanical transmission system falls down. Typically, the mechanical ICE is running at substantially lower efficiencies, 25%. In contrast, the series system can keep the ICE in the peak efficiency modes:
MECHANICAL
(25% ICE) * (95% 1st gear) * (95% 2nd gear) * (95% 3d gear) =
(25% ICE) * (86% three gears) ~= 21%
SERIES ELECTRICAL
(35% ICE) * (90% generator) * (90% converter) * (90% motor) =
(35% ICE) * (73% transfer) ~= 25%
This is one of the reasons we find some hybrid skeptics 'game' informal MPG testing in their articles by using narrow test environments. They also use a manual transmission to avoid torque converter losses. So how does a parallel hybrid electric come out?
In a well designed system, it uses the MECHANICAL efficiencys when they are best and ELECTRICAL efficiencies when they are best. We can actually see this in my MPG distribution data:
That tail on the right edge reflects success in being in the 'sweet spot' of either the electrical or mechanical system efficiencies. It takes planning and attention to detail but it shows the potential of my Classic, 03 Prius. The current Prius has tuned the system to move the peak further to the right, apparently an extra 3
MPH.
IMHO, it makes sense to provide a mechanical path for high-speed ranges because that is where the ICE and transmission efficiencies excel. At lower speeds and under variable speed ranges, the electrical transmission system works best. Although it is harder to design, with good computer control, the vehicle can achieve optimum performance.
IMHO, a parallel system can achieve the best total system performance over those having only a mechanical or electrical transmission system. But it takes attention to detail to make sure the vehicle works in the "best" of each mode across all operating conditions. It means the designers have to abandon the bias of "all problems look like a nail when you have just a hammer."
Bob Wilson
ps. Thanks for moving the thread!