How do hybrids work?
Discussion
All this talk of hybrid vehicles has left me wondering how they work.
I read somewhere that the conventional engine drives a generator which provides additional power for the batteries, and the electric motor does all the driving, but I can't see that being the case with cars like La Ferrari or the McLaren P1.
Presumably in these cases the conventional engine still has a direct mechanical connection to the rear wheels through gearbox and driveshafts. So how does the electric power get applied?
I don't see how an electric motor can operate independently of the conventional engine and still drive the same mechanical components, so I'm presuming that the motor powers the mechanical components by magnetic field?
Would very much appreciate if someone could explain it to me.
I read somewhere that the conventional engine drives a generator which provides additional power for the batteries, and the electric motor does all the driving, but I can't see that being the case with cars like La Ferrari or the McLaren P1.
Presumably in these cases the conventional engine still has a direct mechanical connection to the rear wheels through gearbox and driveshafts. So how does the electric power get applied?
I don't see how an electric motor can operate independently of the conventional engine and still drive the same mechanical components, so I'm presuming that the motor powers the mechanical components by magnetic field?
Would very much appreciate if someone could explain it to me.
Before you can understand the multiple operating modes of a hybrid vehicle, you need to understand how a conventional one works and where it's various in-inefficiencies are.
1) Burning the fuel: All cars "burn" a fuel (where fuel = an convenient energy store) to release power to move the mass of the car around. In a conventional car the Internal Combustion Engine does this. The basic thermal efficiency of this engine is the primary driving force for efficiency. Typically, this is much less than 30% for a passenger car (i.e. >70% of the energy is lost to heat!)
2) Converting this power into tractive effort: All cars must somehow leverage the heat released by a fuel being burn into useful tractive effort. In a ICE vehicle, this is done by the crank and gearbox / final drive system. This also has an efficiency associated with it.
3) Overcoming friction and inertia: All cars have both mechanical friction, aerodynamic drag and a mass. As such, certain amount of power is required to push the car along. In the case of the cars mass, the energy is stored in the vehicles mass as kinetic energy, and lost (to heat in the brakes) when you need to slow down again. The rolling & aerodynamic drag lumped together are the vehicles "roadload" a characteristic curve which defines the power required for the vehicle to maintain any constant speed (on a flat(zero gradient) road).
So, to use less fuel, we have several methods:
a) Reduce the road load: The best option for any type of vehicle. Simply reduce how much power is required for any given speed, via better aerodynamics (lower Cd or frontal area), lower rolling friction (bearings, tyres, brake drag etc).
b) Reduce the vehicle mass: Helps during acceleration (changes of vehicle speed), less KE means less fuel burnt for any change in speed
c) Increase the thermal efficiency of the power plant: Difficult to do, but the biggest loss is still the fundamental thermal efficiency of the fuel -> heat conversion. The best option is to ensure the ICE only operates under speed and load conditions that maximise it's efficiency.
d) Decrease the parastic losses of the powertrain: Any mechanism used to leverage heat into motion has an intrinsic efficiency. Lower losses, by downspeeding the engine (taller gears) or reducing things such as power steering loads (which don't directly drive the vehicle along)
1) Burning the fuel: All cars "burn" a fuel (where fuel = an convenient energy store) to release power to move the mass of the car around. In a conventional car the Internal Combustion Engine does this. The basic thermal efficiency of this engine is the primary driving force for efficiency. Typically, this is much less than 30% for a passenger car (i.e. >70% of the energy is lost to heat!)
2) Converting this power into tractive effort: All cars must somehow leverage the heat released by a fuel being burn into useful tractive effort. In a ICE vehicle, this is done by the crank and gearbox / final drive system. This also has an efficiency associated with it.
3) Overcoming friction and inertia: All cars have both mechanical friction, aerodynamic drag and a mass. As such, certain amount of power is required to push the car along. In the case of the cars mass, the energy is stored in the vehicles mass as kinetic energy, and lost (to heat in the brakes) when you need to slow down again. The rolling & aerodynamic drag lumped together are the vehicles "roadload" a characteristic curve which defines the power required for the vehicle to maintain any constant speed (on a flat(zero gradient) road).
So, to use less fuel, we have several methods:
a) Reduce the road load: The best option for any type of vehicle. Simply reduce how much power is required for any given speed, via better aerodynamics (lower Cd or frontal area), lower rolling friction (bearings, tyres, brake drag etc).
b) Reduce the vehicle mass: Helps during acceleration (changes of vehicle speed), less KE means less fuel burnt for any change in speed
c) Increase the thermal efficiency of the power plant: Difficult to do, but the biggest loss is still the fundamental thermal efficiency of the fuel -> heat conversion. The best option is to ensure the ICE only operates under speed and load conditions that maximise it's efficiency.
d) Decrease the parastic losses of the powertrain: Any mechanism used to leverage heat into motion has an intrinsic efficiency. Lower losses, by downspeeding the engine (taller gears) or reducing things such as power steering loads (which don't directly drive the vehicle along)
Edited by anonymous-user on Sunday 12th January 16:44
Broadly, a typical passenger car spends it's time operating under three "zones":
1) Very low(<15mph) or stationary road speed, engine at idle (or just off). Typical of urban or heavy traffic useage
2) Mid speed operation, lots of stop starts. Typical of extra urban operation, between 15mph and 50mph
3) High speed cruise, >50mph, generally few stops, and often large distances covered.
In zone 1) An ICE vehicle is highly inefficient, because the average road load is often so small (it is zero when stationary!). As such, pretty much all fuel burnt is "lost", either to engine friction, or other parasitic losses (PAS, Alternator, water/oil pump etc)
In Zone 2) the average road load is still quite low (typical car takes only ~10kW to do 50mph), but in stop / start traffic the energy transferred into and out of the vehicles mass as KE is large. Rolling friction is high as losses from tyres etc start to climb with speed. As average "gear" is low, the ratio of engine speed to vehicle speed is also poor, and lots of energy are lost to engine friction.
In zone 3) Aerodynamic loads start to dominate. The vast majority of energy is just being used to push the car through the atmosphere, although tyres losses can be quite high. Engine friction becomes less dominating as generally a tall gear can be used.
1) Very low(<15mph) or stationary road speed, engine at idle (or just off). Typical of urban or heavy traffic useage
2) Mid speed operation, lots of stop starts. Typical of extra urban operation, between 15mph and 50mph
3) High speed cruise, >50mph, generally few stops, and often large distances covered.
In zone 1) An ICE vehicle is highly inefficient, because the average road load is often so small (it is zero when stationary!). As such, pretty much all fuel burnt is "lost", either to engine friction, or other parasitic losses (PAS, Alternator, water/oil pump etc)
In Zone 2) the average road load is still quite low (typical car takes only ~10kW to do 50mph), but in stop / start traffic the energy transferred into and out of the vehicles mass as KE is large. Rolling friction is high as losses from tyres etc start to climb with speed. As average "gear" is low, the ratio of engine speed to vehicle speed is also poor, and lots of energy are lost to engine friction.
In zone 3) Aerodynamic loads start to dominate. The vast majority of energy is just being used to push the car through the atmosphere, although tyres losses can be quite high. Engine friction becomes less dominating as generally a tall gear can be used.
So, if we are talking about hybrid vehicles, which take there entire propulsive effort from burning fuel in an onboard ICE (as opposed to say a plug in hybrid that also takes electricity from the grid) we need to ensure that the fuel is only converted to useful work at the highest possible efficiency.
For an ICE, there is really a very narrow window of operating speed and load at which the most energy possible is leveraged from a unit of fuel being burnt. If you Google Brake Specific Fuel Consumption (BSFC) you will find "maps" of engine speed and load that indicate how efficient the engine is at doing just that. Th evast majority of hybrids are simple a means to this end. i.e. only run the engine at a condition where we get the most out of the fuel!
The problem of course, is that this isn't very practical! Who wants a car that can travel at only two speeds, zero and probably about 40mph. That would be somewhat inconvenient! As a result electric traction systems have been developed to try to "buffer" the ICEs fuel->energy conversion, so that the car can still operate at a wide range of loads and speeds, but the engine can be left operating at just one load/speed.
Now, depending the vehicles roadload, the mode of operation of the hybrid system will need to vary:
1) Zero / Low speeds: Roadload is tiny, so engine off, use the energy in the battery storage system. Only start the engine when absolutely necessary, and then operate it at it's most efficient load/speed to recharge the battery. In effect, this is the ultimate expression of "downspeeding" or a theoretically infinite top gear ratio if the engine is off!.
2) Medium speeds: Still a low average roadload, but transient spikes in power might be necessary to accelerate the vehicle as necessary. Start the engine only if required, use it to assist the electric powertrain. Also, use the electric powertrain to harvest vehicle KE when we slow down, rather than waste the energy to heat in the friction brakes!
3) High speeds: Roadload is high, and often we actually have a combination of vehicle speed and engine speeds that mean we can link the engine directly to the wheels (as in a conventional transmission). The benefit of this is super low drivetrain losses as we don't need any energy conversion stages between engine flywheel and the road. If the roadload isn't quite enough to get into the high efficiency zone of operation, we can still run the engine at high load (high efficiency) and simply use the electric power to absorb the "extra" (to prevent vehicle accelerating any more) and charge the battery system ready for the next low speed operation zone.
Of course, in reality, environmental conditions, driving style and conditions, vehicle loading, and a million other parameters all effect the roadload at which any given mode is the most efficient to use! This makes the calibration of hybrid cars very time consuming at complex...........
For an ICE, there is really a very narrow window of operating speed and load at which the most energy possible is leveraged from a unit of fuel being burnt. If you Google Brake Specific Fuel Consumption (BSFC) you will find "maps" of engine speed and load that indicate how efficient the engine is at doing just that. Th evast majority of hybrids are simple a means to this end. i.e. only run the engine at a condition where we get the most out of the fuel!
The problem of course, is that this isn't very practical! Who wants a car that can travel at only two speeds, zero and probably about 40mph. That would be somewhat inconvenient! As a result electric traction systems have been developed to try to "buffer" the ICEs fuel->energy conversion, so that the car can still operate at a wide range of loads and speeds, but the engine can be left operating at just one load/speed.
Now, depending the vehicles roadload, the mode of operation of the hybrid system will need to vary:
1) Zero / Low speeds: Roadload is tiny, so engine off, use the energy in the battery storage system. Only start the engine when absolutely necessary, and then operate it at it's most efficient load/speed to recharge the battery. In effect, this is the ultimate expression of "downspeeding" or a theoretically infinite top gear ratio if the engine is off!.
2) Medium speeds: Still a low average roadload, but transient spikes in power might be necessary to accelerate the vehicle as necessary. Start the engine only if required, use it to assist the electric powertrain. Also, use the electric powertrain to harvest vehicle KE when we slow down, rather than waste the energy to heat in the friction brakes!
3) High speeds: Roadload is high, and often we actually have a combination of vehicle speed and engine speeds that mean we can link the engine directly to the wheels (as in a conventional transmission). The benefit of this is super low drivetrain losses as we don't need any energy conversion stages between engine flywheel and the road. If the roadload isn't quite enough to get into the high efficiency zone of operation, we can still run the engine at high load (high efficiency) and simply use the electric power to absorb the "extra" (to prevent vehicle accelerating any more) and charge the battery system ready for the next low speed operation zone.
Of course, in reality, environmental conditions, driving style and conditions, vehicle loading, and a million other parameters all effect the roadload at which any given mode is the most efficient to use! This makes the calibration of hybrid cars very time consuming at complex...........
So, now knowing all that, how do we design a hybrid system that results in covering the largest possible operating envelope with the highest average fuel efficiency, whilst also juggling the opposed factors of complexity and cost?
Well, that is, as they say, the billion dollar question ;-)
Well, that is, as they say, the billion dollar question ;-)
McWigglebum4th said:
Not all hybrids are the same
The prius is about the cleverest one out there
The prius is about the cleverest one out there
This link
http://prius.ecrostech.com/original/PriusFrames.ht...
is quite old, and some of the links don't work any more, but the basic principles are still valid, and it's IMHO a really good introduction to the Prius drivetrain.
The systems in high performance cars like the P1 are different, since they use the hybrid system mainly to improve acceleration rather than improve fuel efficiency. For example, I believe that the P1 does not use regenerative braking, since the view was that it would compromise the feel of the car.
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