View Full Version : "Serial Hybrid" cars

Dr Mordrid
31st December 2006, 09:31
First a recap;

Parallel Hybrids:


In a parallel hybrid both the electric and internal combustion engine are attached to the drive line with the controller switching between them as needed. A large LiIon battery pack is used, which adds cost, weight, charging and cooling requirements. Most of todays hybrids are parallels.

Plug-In hybrids:

A plug-in hybrid adds the ability to recharge the battery pack overnight by plugging into the house lines.

Now for the meat....

Serial hybrids:


These differ in that they are much less complex mechanically, require smaller battery packs and engines, making them potentially much cheaper than todays hybrids.

In a serial hybrid only the electric drive is connected to the driveline with a much smaller internal combustion engine driving the generator. This could be as small as a 10-50 hp gas/diesel piston, rotary or even a small gas turbine run only for drives longer than the batteries can handle or when more power is required.

Since the batteries are only used for short drives and bursts lead-acid or NiMH packs smaller than todays LiIon packs can be used to reduce costs, or not.

With NiMH's the "no start" range could reach 30+ miles, enough for most peoples daily driving, but at the end of this range the internal combustion engine kicks in to power the car and recharge the batteries.

Another type of serial hybrid adds a "Peak Power Unit" (PPU) for even more burst power and braking regeneration. This can be a flywheel or ultracapacitor.


In either case the fuel costs, maintenance costs, weight, and complexity are massively reduced over todays hybrids.

Toyota's Synergy Drive:


This "Combined Hybrid" incorporates features of both parallel and serial, which means it's complex and expensive.


Look for info on serial hybrids at the upcoming North American Auto Show here in Detroit, likely from GM and perhaps a few others.

GM is definitely serious about them. Expect a prototype early this year if not at the NAAS.

31st December 2006, 15:23
remove the battery and a serial hybrid is not so different to a diesel train...
As for now, I still survive without a car here, I would like to purchase one in the near future just for getting away on weekends, I will be looking at hybrids. sadly plug in hybrids are not really viable in this country, as most of us don't have a gargage, but park our cars in a neighboring car park.

31st December 2006, 15:24
This type of hybrid has been covered in a couple of other threads earlier in the year, but not so directly: The "serial" model is the most efficient long-term hybrid, and actually my "favorite" concept. Brian Ellis terms these types of hybrids "Diesel-Electrics" which is exactly what they would be.

The biggest advantage of this type of hybrid is the lack of the need for a broad RPM range for the engine to operate within... it is far easier to design, build and maintain a constant RPM engine that it is to build a Variable RPM ICE connected to some sort of a transmission system. It almost goes without saying that forecasting routine maintenance and wear checks on these types of engines is quite simple.

Most of the prototypes I have seen activate the ICE whenever the A/C or heating systems are needed (and may need to be engaged anyway to keep the battery temperatures within norms in cold-weather environments)

This is not a new concept at all: Nearly every modern cruise ship in the world uses a diesel-electric or a GasTurbine-electric power system (sans the batteries for the propulsion engines); but for conventional submarines, this has been the standard propulsion method from about 1915 until today.

Dr Mordrid
31st December 2006, 16:45
Yup, but now the carmakers are finally getting serious about them.

About damn time IMO.

31st December 2006, 20:34
Lead-Acid Batteries can be used in this type of a car with pretty good efficiency, they may be heavy, but lead-acid battery capacity and power delivery are fairly good with thin plate cells. Lead-acid technology, including the recycling technology, and their behavior in a wide variety of environments is well-understood.

Eco-friendly does not necessarily mean you stop using hazardous materials: you find ways to use those types of materials in a safe and responsible manner.

The biggest issue with the Hybrids, particularly the Electric-only and Diesel-electrics, are how the power cells react in an accident; some of the more advanced batteries can be downright lethal to be around (Sodium-Bromide Cells), and Fuel cells have a LOT of potential energy stored in extremely reactive compounds. Newer batteries have even higher energy densities; You do not want to be around them if they are cracked open or dead-shorted internally.

Lead Acid batteries are no exception, but rescue crews already know how to deal with lead-acid batteries. Manufacturers are already working on better safety mechanisms and fail-safe designs to prevent catastrophic battery failures. Sulfuric acid is nasty, but nothing that a box of baking soda can't fix: the same thing cannot be said for some other battery types.

Right now, most Electrics are using efficient AC motors; quite a bit of power is lost converting DC to AC through the most efficient Inverter. Straight DC motors have some interesting possibilities in this regard. I've seen some high duty-cycle DC motors normally used for Automatic Screw Machines which would scale nicely to 50HP/Wheel. They even had self-cleaning armatures and brush wear sensors built into them.

Brian Ellis
1st January 2007, 01:39
The BIG disadvantage of the purely electric drive is the size and weight of the motors. The advantage is that, unlike your diagrams, you can do away with the differential by having separate inboard motors for each of the driving wheels and rotational speed detectors so that the computer can dose the power to each motor. But to have motors totalling, say, 150 kW peak with a 30 min duty cycle at near-full power (capable, for example, to get you over the top of the St Bernard Pass) and you are looking at a VERY heavy vehicle. Ideally, you could have the motors in the wheels, but the unsprung weight would make the ride very uncomfortable.

Toyota are working on a variant of their Synergy drive which sounds quite interesting but I think may not be suitable for Prius-style cars but more for their Lexus range. The big problem with the Prius is the highway consumption, although it has the urban consumption to a fine art. This is the opposite of the Honda Civic hybrid, with its simpler drive train. The Prius electric motor is designed for highest efficiency at a low-revs high-torque situation. The new concept is to have two motors, one like the current one and the other designed for max efficiency at a high-revs low-torque situation and to have a three-way mechanical power selector, instead of the current two-way one. This would mean that the fuel motor would not have to run permanently at speeds over ~80 km/h. I hate to think of the cost, though.

As explained in an earlier thread, I cannot believe in EV or plug-in hybrids becoming mainstream for lack of electricity infrastructure in residential areas in most countries, not to mention the cost of batteries. For me, the only EV solution and it could possibly become half-stream in many countries, is the interchangeable battery type. You go to a filling station, whip out a kind of palletted plug-in battery and replace it with a fully charged one (two minute job), like you do with LPG cylinders now. It would be easier and cheaper to supply the filling station with the electric infrastructure to charge, say, 1,000 batteries/day, using available surplus electricity 24/7 than it would 1,000 homes scattered over 30 or 40 km of roads in a residential area. However, this assumes adequate generating capacity, which is not always evident.

1st January 2007, 02:50
I don't think we'll see an electric motor in-hub anytime soon: Though it is a possibility if you use a DC motor with an Electromagnet instead of a static permanent magnet. The big downside to this arrangement (aside from unsprung weight) is the multiple power connections in a fairly exposed part of the vehicle (the wheel wells).

More likely are inboard motors connected to the wheels via CV Joints. The mechanical losses are almost trivial compared to the improvements in ride and overall balance of the vehicle.

Another possibility (if a bit of a reach) is the hydraulic drive circuit: this is quite "lossy" mechanically speaking (From my last reading about it...about 20 years ago), but it does have a number of good points going for it when you look at it from an Electrically-powered perspective. It would allow a single central electric motor to supply each wheel independently, and would be very simple to regulate torque and speed to each wheel. It would also allow putting the drive units into the hub of each wheel with very little weight cost.

The high use Diesel-electric hybrid could accomplish recharging it's cells while unattended: The vehicle would continue to run it's ICE in a key-off condition until the batteries were charged - this would have to be accompanied by a pretty snazzy system of checks to make sure it wasn't parked in a sealed garage - a Carbon Monoxide detector would probably the cornerstone of such a system.

3rd January 2007, 14:23
Yup, but now the carmakers are finally getting serious about them.

About damn time IMO.

About damn time you say? There have been competitions in Japan about fuel efficient cars/engines for years now. What we are starting to see in North America is what they have already been using seeing for years.

But I like the serial design with the flywheel/capacitor. To me it would seem more fuel efficient than Toyota's Synergy Drive.