Hybrid vehicle: series or parallel drive. Which is better? 1

[PatentPro]

PatentPro writes:

I believe that the era of the hybrid vehicle has arrived. Internal combustion engines always operate inefficiently under high loads near the engine's rated power: eg climbing hills, accelerating. But an electric motor, properly sized, can perform efficiently under these circumstances.

The Prius is a parallel drive system. The gasoline motor and electric motor can operate in parallel. The Volt is a series drive system: the internal combustion engine only sees the generator.

Who's right: Toyota or General Motors?

 

[BrianPetersen]

Wutzu, unless the driving patterns of the Formula Hybrid test are designed to favor series operation ... I think the parallel drivetrains will be the ONLY ones that remain competitive.

Whenever the engine is running, its power is best used by going gear-to-gear to the wheels. It requires operating the engine somewhat off its absolute peak efficiency single operating point, but the loss by doing so is (in most cases) not as great as the loss of generating electricity, rectifying, storing, inverting it, and using it to power a motor.

If the test sequence takes start/stop and low-speed operation and extended periods of zero (stopped) to an extreme then MAYBE the series method MIGHT work out better.

 

[wutzu]

BrianPeterson,

You raise great points. I

The competition, as I see it, is biased toward series hybrids. The big kickers are 5% of competition points for integration of gas and electric systems, and 7.5% of competition points for electric-only acceleration. If it weren't for these events and the hybrid-electric entry requirement, the winning car would be a 310cc turbodiesel, or a 250cc naturally aspirated gas engine.

The a mechanical transmission attached to an engine running at its max efficiency point is obviously the most efficient power option-assuming you keep the engine at that efficiency point. Since most of the events incorporate frequent acceleration/deceleration, I don't think the engine can be kept at that point.

A series hybrid isn't as efficient with its energy conversions since energy goes from the generator to an inverter to batteries to a controller to a motor. However, the generator can be much smaller, because its power requirements are buffered out by the batteries (think 2kW continuous instead of 20kW at peaks). The generator can also operate at its max efficiency point, which mitigates some of the energy conversion losses.

The biggest advantage that I see series hybrid having is that it can be built much lighter. A small generator driving batteries driving a motor is lighter weight than a large engine driving a transmission, but also hauling along the requisite motor and batteries/supercapacitors. Then add in the somewhat more complicated packaging issues of a parallel system and the resultant weight gain.

 

[wutzu]

The train motor is a bad comparison. The Cummins ISX 385 is a diesel engine weighing 1.5 tons, putting out 385 horsepower. That sounds like it would be terrible if it were scaled down to fit a Volkswagen TDI. But it's not, because it's not the same sort of engine. It puts out 385hp, but 1450lbf-ft of torque at 2k rpm. It's built for torque to haul large loads up hills.

The reason the train's traction motor is so heavy is that it needs the copper and magnets to generate massive amounts of torque. The torque is proportional to the current through the motor coils, which is limited by the wire thickness. If we scaled that down to fit a car, it would only be good for tractors or people who drove slowly. But nobody does, they use smaller motors designed for less current (torque) but higher rpms, which comes out to more horsepower at the top end.

 

[BrianPetersen]

Show me a lightweight AC motor that achieves the 98.4% efficiency that someone else quoted. In every case that I know of, and by NO means do I claim to know every possibility, high motor/generator efficiency is achieved by using lots of iron and copper.

Certainly automotive-scale electric motors would be designed lighter than locomotive AC traction motors - at a cost of several points of efficiency. There is no choice but to do so. But ... if additional magnetic hysteresis and eddy current and other losses caused by using less iron and higher AC frequencies don't get you, windage and having to use additional gear reduction will.

Regarding that competition, a test for maximum electric-only acceleration does bias things towards having a big drive motor and high-current-capacity batteries and inverters. But that doesn't necessarily imply a series design. You can use a small combustion engine capable of being mechanically connected with power rating sized for cruising in the best efficiency range, and a big electric motor for acceleration. If that sounds familiar, it's not too different from how a Prius (or Honda Insight) is designed!

 

[GregLocock]

CSIRO motor

http://www.csiro.au/files/CSIROau/Software/SolarCar.zip

8 kW thermal limit, 1.2 kg copper, 4.8 kg magnets.


Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BrianPetersen]

That's interesting. But A$12,000 for a motor kit? Ouch!

 

[mheslep]

Three phase AC induction motors reach power density of 4kw/kg (2.4 hp/lb).
See, e.g., AC Propulsion's 3phase/4 pole:

http://www.acpropulsion.com/products-drivesystem.html

High power density at high power levels is simply not common (yet), because high power is commonly used where mass is not a design driver (factory floor, locomotive).

I am unaware of any combustion engine design (perhaps something experimental?) that touches 4kw/kg, short of the gas turbine at 8kw/kg, and it appears high temperature super conductor electric motors will exceed even that.

 

[mheslep]

Note Tesla gives the efficiency of that motor (modified by them I believe) as 92% average, 85% at peak power.

http://www.teslamotors.com/performance/tech_specs.php

 

[GregLocock]

Brian - production version of that motor would have been around $500 10 years ago, there are a couple of good reasons to think it will suit certain niches, at much higher power ratings, but they have not been announced yet.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Surestick]

The big advantage I see with the Volt (and granted it's only one specific version of a series hybrid drivetrain) is that for the first 40 miles it's purely electric and therefore guaranteed more fuel efficient than the Prius.
I recall reading somewhere that the average daily commute was 30 miles in a day so for the average person (with 2.5 kids!) they may rarely have to fill up.

Of course that's fuel burned from the tank efficiency not fuel burned at the power station efficiency and that's assuming it was charged, the driver doesn't have a lead foot, etc.
Also a quick google search finds no solid info on how they plan to provide heat in the winter and how that will affect fuel use or range.