Long time EV enthusiast here, if you didn't know.
So Ive been thinking a lot lately about hybrids and EVs as I drive my Prius around town installing AC & DC drives and such. I finally found the time to sit down and put all my ideas down in writing but found it a little hard to organize them into a linear thought process or exercise any brevity, so if you have any patience left in you at this hour, please read and critique.
I was thinking that gasoline powered automobiles are so inefficient for 2 reasons: (1) The engines must be sized for worst case scenario. I have read that it only takes ~20HP to drive full sized sedan down the highway. This could be accomplished with a single cylinder lawn mower engine, yet the sedan needs 5 extra cylinders and 200 extra HP so that the driver can achieve that 70MPH in haste, without being ran down by semi trucks. So once at freeway speeds, we have 5 cylinders banging away for no reason. I have read that some cars were built to use less than their full complement of cylinders when not needed, but that was a big failure so it hasnt been tried again. (2) I have read that internal combustion engines have a peak theoretical efficiency of ~32%. That sucks, but it gets even worse; that 32% is only approached at a specific speed and torque, known as the peak of the power band. I will refer to this peak as peak efficiency henceforth. At all other speeds and torque loads (accounting for almost all normal driving conditions) the efficiency is much lower than the peak efficiency.
So my idea is that the hybrid vehicle should be purely electric drive, with a capacitor bank instead of a battery bank. The capacitor bank should be sized such that at nominal voltage, it will provide only enough power for one pedal to the floor acceleration uphill with full cargo load to freeway speed. The internal combustion engine should be sized only large enough to slowly recharge the cap bank after said heavy acceleration, once at freeway speed. Using the 20HP for a full sized sedan rectally retrieved number as an example, perhaps the engine could be sized around 40HP.
The engine would be ran at a fixed RPM and coupled to a DC generator. A generator with a fixed field supply and ran at a fixed RPM would act as a constant voltage supply. Charging the cap bank with this constant voltage supply would result in a high load at the beginning of the charge, and tapering off to very little load once the bank approaches nominal voltage - allowing the engine to run outside it's peak efficiency . My idea is to intelligently regulate the field current to have the effect of turning the generator into a constant current source instead of a constant voltage source. Lets say we have a cap bank capable of 400V, and we establish 200V as the nominal cap bank voltage. We establish 150V as the lower limit (generator turn on) and 300V as the upper limit (generator turn off). We could regulate the field for a desired actual + 20V (rectally retrieved number) output. So when the voltage present across the bank is 160V, the generator attempts to output 180V, and at 161V it attempts to output 181V, and so on until the actual bank voltage is 300V, at which point it shuts off. We reserve the remaining 100V for any energy that may be recouped by regenerative braking. By turning our generator into a constant current supply in this manner, and by fine-tuning the RPM and actual + X volts, (AKA current, AKA torque) we provide what appears to the internal combustion engine as fixed load that is exactly at the very peak of its power band (peak efficiency) 99% of the time. Also, the electric machine could be used as a starter for the engine.
So now on the traction motor side of things, we have a wildly fluctuating supply that could be as low as 150V or as high as 400V. How do we cope with that? Well, 2 things come to mind. We can employ the same intelligent field current regulating techniques as the generator when below rated voltage, and PWM current limiting when above. Lets say we use a DC motor rated for 200V. When the available voltage is 150V, we can prematurely (below base rated speed) employ field weakening to achieve desired speed with less than desired voltage. This will be at the expense of higher than desired current draw, but this shouldnt be a problem for the cap bank to supply, and as long as we have adequate cooling and high quality brushes, it shouldn't be a problem at all. When the voltage is 400V, 50% duty cycle PWM should have us right on the money. A combination or varying degree of these 2 techniques could be tailored for any possible bank voltage.
All this talk of intelligent field control leads me to regenerative braking. In the same way that we control the mechanical torque load applied to the generator by setting the target voltage at a prescribed voltage over and above cap bank voltage, upon braking, we can turn the traction motor into a generator and let the brake pedal set the prescribed X volts above bank voltage (AKA current, AKA torque). I believe that regenerative braking employed in this manner could provide significant braking power and power recovery, able to slow the car almost as well as mechanical brakes. Mechanical brakes would be engaged only in emergency circumstances, or when approaching zero speed, or when cap bank voltage is > 400V. Ordinary electrics and hybrids have batteries, which have a low do not exceed charge rate and therefore lose out on a lot of potential energy recovery when braking; with the cap bank, I think it would swallow whatever the regenerating motor could spit out.
The forward/reverse functions could be controlled by a low-power H-bridge on the field supply, minimizing losses. The armature could be switched by a single transistor instead of the 6 required for a 3 phase motor, again minimizing electrical losses. The high power regenerative braking could reclaim a much higher quantity of energy than what my Prius can. I have seen DC motors advertised with high-90s percent efficiency. The field control switching in both the motor and the generator can be considered almost negligible, so I tally up very little losses in the electrical side of the house, and with the engine being several cylinders smaller than whats found in a similarly sized internal combustion engine car, and ran exclusively at its peak efficiency, I foresee this being a VERY efficient car. The bulk of the electrical inefficiency would probably be in the step-down DC/DC converter for powering 12VDC loads. But then I am also dreaming up ideas to all-but eliminate the 12V circuit and power the larger accessories (A/C, lights, power steering, etc.) directly from the fluctuating bank voltage.
One down side I can think of, if you came out to your car to leave for work in the morning and found the cap bank at 0V, you might have to wait a little while in the driveway for it to charge before you take off. BUT, this could mitigated by a float charger that plugs into 120V power. 120V>step up transformer>rectifier, current limiter = low power 300V top-off. In the rare circumstance where maybe you went camping for a few days and your bank bled down and theres nowhere to plug in, I suppose a 12V starter and backup battery wouldnt hurt.
So, why is it not done this way? Why does my Prius have a 1.8L engine (same size as comparable non-hybrid), and why is my 1.8L engine coupled to the wheels mechanically with some convoluted sun gear and running outside its peak efficiency? Why am I carrying around 800Lbs worth of batteries when I could be carrying 100Lbs of capacitors? Why is it a $10,000 upgrade to get a cord I plug the thing in at night? Why does my Prius engage the mechanical brakes every time I come to a stop instead of recouping the dollars I invested to get it up to freeway speed?
I know, I know, everyones a critic. It takes teams of brilliant engineers to design these things and its not easy, and then everyone and their mother out there thinks they can do it better. But thats why I started the discussion, to find out why my way isnt better. Im just here to learn.
So Ive been thinking a lot lately about hybrids and EVs as I drive my Prius around town installing AC & DC drives and such. I finally found the time to sit down and put all my ideas down in writing but found it a little hard to organize them into a linear thought process or exercise any brevity, so if you have any patience left in you at this hour, please read and critique.
I was thinking that gasoline powered automobiles are so inefficient for 2 reasons: (1) The engines must be sized for worst case scenario. I have read that it only takes ~20HP to drive full sized sedan down the highway. This could be accomplished with a single cylinder lawn mower engine, yet the sedan needs 5 extra cylinders and 200 extra HP so that the driver can achieve that 70MPH in haste, without being ran down by semi trucks. So once at freeway speeds, we have 5 cylinders banging away for no reason. I have read that some cars were built to use less than their full complement of cylinders when not needed, but that was a big failure so it hasnt been tried again. (2) I have read that internal combustion engines have a peak theoretical efficiency of ~32%. That sucks, but it gets even worse; that 32% is only approached at a specific speed and torque, known as the peak of the power band. I will refer to this peak as peak efficiency henceforth. At all other speeds and torque loads (accounting for almost all normal driving conditions) the efficiency is much lower than the peak efficiency.
So my idea is that the hybrid vehicle should be purely electric drive, with a capacitor bank instead of a battery bank. The capacitor bank should be sized such that at nominal voltage, it will provide only enough power for one pedal to the floor acceleration uphill with full cargo load to freeway speed. The internal combustion engine should be sized only large enough to slowly recharge the cap bank after said heavy acceleration, once at freeway speed. Using the 20HP for a full sized sedan rectally retrieved number as an example, perhaps the engine could be sized around 40HP.
The engine would be ran at a fixed RPM and coupled to a DC generator. A generator with a fixed field supply and ran at a fixed RPM would act as a constant voltage supply. Charging the cap bank with this constant voltage supply would result in a high load at the beginning of the charge, and tapering off to very little load once the bank approaches nominal voltage - allowing the engine to run outside it's peak efficiency . My idea is to intelligently regulate the field current to have the effect of turning the generator into a constant current source instead of a constant voltage source. Lets say we have a cap bank capable of 400V, and we establish 200V as the nominal cap bank voltage. We establish 150V as the lower limit (generator turn on) and 300V as the upper limit (generator turn off). We could regulate the field for a desired actual + 20V (rectally retrieved number) output. So when the voltage present across the bank is 160V, the generator attempts to output 180V, and at 161V it attempts to output 181V, and so on until the actual bank voltage is 300V, at which point it shuts off. We reserve the remaining 100V for any energy that may be recouped by regenerative braking. By turning our generator into a constant current supply in this manner, and by fine-tuning the RPM and actual + X volts, (AKA current, AKA torque) we provide what appears to the internal combustion engine as fixed load that is exactly at the very peak of its power band (peak efficiency) 99% of the time. Also, the electric machine could be used as a starter for the engine.
So now on the traction motor side of things, we have a wildly fluctuating supply that could be as low as 150V or as high as 400V. How do we cope with that? Well, 2 things come to mind. We can employ the same intelligent field current regulating techniques as the generator when below rated voltage, and PWM current limiting when above. Lets say we use a DC motor rated for 200V. When the available voltage is 150V, we can prematurely (below base rated speed) employ field weakening to achieve desired speed with less than desired voltage. This will be at the expense of higher than desired current draw, but this shouldnt be a problem for the cap bank to supply, and as long as we have adequate cooling and high quality brushes, it shouldn't be a problem at all. When the voltage is 400V, 50% duty cycle PWM should have us right on the money. A combination or varying degree of these 2 techniques could be tailored for any possible bank voltage.
All this talk of intelligent field control leads me to regenerative braking. In the same way that we control the mechanical torque load applied to the generator by setting the target voltage at a prescribed voltage over and above cap bank voltage, upon braking, we can turn the traction motor into a generator and let the brake pedal set the prescribed X volts above bank voltage (AKA current, AKA torque). I believe that regenerative braking employed in this manner could provide significant braking power and power recovery, able to slow the car almost as well as mechanical brakes. Mechanical brakes would be engaged only in emergency circumstances, or when approaching zero speed, or when cap bank voltage is > 400V. Ordinary electrics and hybrids have batteries, which have a low do not exceed charge rate and therefore lose out on a lot of potential energy recovery when braking; with the cap bank, I think it would swallow whatever the regenerating motor could spit out.
The forward/reverse functions could be controlled by a low-power H-bridge on the field supply, minimizing losses. The armature could be switched by a single transistor instead of the 6 required for a 3 phase motor, again minimizing electrical losses. The high power regenerative braking could reclaim a much higher quantity of energy than what my Prius can. I have seen DC motors advertised with high-90s percent efficiency. The field control switching in both the motor and the generator can be considered almost negligible, so I tally up very little losses in the electrical side of the house, and with the engine being several cylinders smaller than whats found in a similarly sized internal combustion engine car, and ran exclusively at its peak efficiency, I foresee this being a VERY efficient car. The bulk of the electrical inefficiency would probably be in the step-down DC/DC converter for powering 12VDC loads. But then I am also dreaming up ideas to all-but eliminate the 12V circuit and power the larger accessories (A/C, lights, power steering, etc.) directly from the fluctuating bank voltage.
One down side I can think of, if you came out to your car to leave for work in the morning and found the cap bank at 0V, you might have to wait a little while in the driveway for it to charge before you take off. BUT, this could mitigated by a float charger that plugs into 120V power. 120V>step up transformer>rectifier, current limiter = low power 300V top-off. In the rare circumstance where maybe you went camping for a few days and your bank bled down and theres nowhere to plug in, I suppose a 12V starter and backup battery wouldnt hurt.
So, why is it not done this way? Why does my Prius have a 1.8L engine (same size as comparable non-hybrid), and why is my 1.8L engine coupled to the wheels mechanically with some convoluted sun gear and running outside its peak efficiency? Why am I carrying around 800Lbs worth of batteries when I could be carrying 100Lbs of capacitors? Why is it a $10,000 upgrade to get a cord I plug the thing in at night? Why does my Prius engage the mechanical brakes every time I come to a stop instead of recouping the dollars I invested to get it up to freeway speed?
I know, I know, everyones a critic. It takes teams of brilliant engineers to design these things and its not easy, and then everyone and their mother out there thinks they can do it better. But thats why I started the discussion, to find out why my way isnt better. Im just here to learn.