The Electric Car
in [Design]
|
From: MooseFET on 2 Aug 2007 09:13 On Aug 1, 11:24 pm, JosephKK <joseph_barr...(a)sbcglobal.net> wrote: > Nobody nob...(a)nowhere.com posted to sci.electronics.design: > > > > > On Thu, 02 Aug 2007 03:53:28 +0100, Eeyore wrote: > > >>> > In city driving it's regenerative braking that can make a huge > >>> > difference. The complexity of shoving electrical and ICE > >>> > motive power through some combined transmission seems plain > >>> > daft though. The series hybrid (in which the ICE simply > >>> > recharges a battery) seems far more sensible all round. > > >>> No, I disagree. The dual electrical machine design beats the > >>> series > >>> system hands down. Having the engine go straight to the wheels > >>> when it makes sense to do so makes the demand on the electrical > >>> system way less. > > >> Why's that an advantage ? It also means you can't have > >> 'meaningful' true electric only operation of it, plus it > >> requiresa gearbox which otherwise may not be needed at all. > > > The disadvantage of an all-electric or series hybrid vehicle is > > that the maximum total power is constrained by both the ICE and > > the electrical system, while a parallel hybrid can potentially > > combine the two, or at least provide a more powerful ICE for > > motorway cruising without having to size the electrical components > > to match. > > > A secondary consideration is that the generator losses subtract > > from any efficiency advantages of the electrical drivetrain. > > > But mostly it's a case of having to size the electrical system to > > the power requirements for motorway cruising even though it has > > fewer advantages over a mechanical drivetrain in that context > > (i.e. regenerative braking is of no benefit). > > NO! The electric drive train has to be sized to the maximum > (majority electrical contribution) acceleration in all cases. It > must be sized (possibly much) larger than the ICE. Toyota seems to agree that the dual electrical machine design needs less hardware to get the job done. I think this is because the electricals only need to provide the acceleration at low speeds. This reduces the peak power that must be delivered by the electrical system.
From: MooseFET on 2 Aug 2007 09:17 On Aug 1, 10:52 pm, Nobody <nob...(a)nowhere.com> wrote: > On Wed, 01 Aug 2007 18:55:41 -0700, MooseFET wrote: > >> For a given shape, at automotive speeds, I think there is a square law > >> relationship between air speed and drag. > > > No thats a cubic law. The energy per mile is the square law one. > > Energy = force * distance. Drag force is proportional to speed squared, so > energy per unit distance is proportional to speed squared. Yes you are right. It was a short circuit between the headphones. I saw "drag" and somehow read "horse power".
From: Guy Macon on 2 Aug 2007 09:37 MooseFET wrote: >Toyota seems to agree that the dual electrical machine design needs >less hardware to get the job done. I think this is because the >electricals only need to provide the acceleration at low speeds. This >reduces the peak power that must be delivered by the electrical >system. Toyota choose the electrical / internal combustion switchable design, but it has not been established that they did so because that design needs less hardware to get the job done. It could be that that design is easy to build on existing assembly lines, or because of some other reason. Nobody has yet produced a car using the locomotive topology; an IC engine driving an electric generator whose output provides power to traction motors with no mechanical connection between the ICE and the wheels -- in essense an electric car that incorporates its own generating station. There are persistant rumors that the upcoming Subaru system will use this scheme. We know that such a system can be made to be very fuel efficient indeed in a locomotive; the question is whether this advanage will remain in an automobile. One difference would be that the automobile will likely have batteries to even out the load between accellerating and coasting. A locomotive-style auto would have some interesting characteristics; at all but the very slowest speed it outputs close to constant power, so the torgue at the driving wheels will tend to inversely vary with speed. A motor-per-wheel system with the proper electronic control would be easy to configure with traction control, anti-lock baraking, and regenerative braking. -- Guy Macon <http://www.guymacon.com/>
From: Eeyore on 2 Aug 2007 09:41 MooseFET wrote: > On the highway, the engine gets connected directly to the wheels. > This takes the sosses of the electric machines out of the picture. What losses ? Electric motors are highly efficient. > It also allows the engine and both motors to be used for passing power. At the cost and complexity of requiring a transmission. I just don't see any sense in it. Graham
From: Nobody on 2 Aug 2007 10:23
On Thu, 02 Aug 2007 08:41:51 +0100, Eeyore wrote: >> >> No, I disagree. The dual electrical machine design beats the series >> >> system hands down. Having the engine go straight to the wheels when >> >> it makes sense to do so makes the demand on the electrical system way >> >> less. >> > >> > Why's that an advantage ? It also means you can't have 'meaningful' true electric only >> > operation of it, plus it requiresa gearbox which otherwise may not be needed at all. >> >> The disadvantage of an all-electric or series hybrid vehicle is that the >> maximum total power is constrained by both the ICE and the electrical >> system, while a parallel hybrid can potentially combine the two, or at >> least provide a more powerful ICE for motorway cruising without having to >> size the electrical components to match. > > At the cost of a vastly more complicated drive train. But they have decades of experience of making those, along with the designs, the tooling, the network of maintenance garages and spare parts for when the mechanics wear out. I suspect that a large proportion of the mechanical drivetrain is taken from a similar pure-ICE vehicle. >> A secondary consideration is that the generator losses subtract from any >> efficiency advantages of the electrical drivetrain. > > With a plug-in hybrid you may not even need to use the ICE/generator on shorter journeys. > Hence no additional losses at all. If that's the normal case, then it's debatable whether it's worth including the ICE/generator at all. If it isn't the normal case, then the efficiency is significant. >> But mostly it's a case of having to size the electrical system to the >> power requirements for motorway cruising even though it has fewer >> advantages over a mechanical drivetrain in that context (i.e. regenerative >> braking is of no benefit). > > Motorway cruising does not require high power. It's acceleration that requires high power. > You can cruise at motorway speeds with say 30kW. It's cruising that requires the power. Acceleration requires the most torque, but not necessarily the power. Also, cruising requires that power continuously, whereas acceleration only requires it intermittently. There's a difference between sizing a motor for 30kW continuous power and for 30kW bursts (especially regarding cooling). Personally, I feel that the series hybrid can be made viable, but it's going to take time and effort. So far, research into electric vehicles has been a drop in the ocean compared to that on ICEs. OTOH, by the time that the output side (motor, power electronics) has matured, the landscape on the generation side may have changed considerably (e.g. fuel cells). |