Take a Drive With Me

I published this piece in 2007 and sent a copy of it to Mazda’s west coast design center.  They are now making a very similar car to the one I describe here. It is a pleasant fantasy to think that I might have had some influence on their design.

Take a drive with me in my new hybrid vehicle.  Well, this vehicle doesn’t exist yet, except in my imagination.  Before we jump in and take off on this imaginary ride, let me describe my vision.

This car is built by Mazda.  It has a rotary (Wankel) engine.  It’s a very simple device, simpler than the ones Mazda put in their RX-7 sports cars so successfully for many years.  Also, it’s a diesel…yes, a diesel rotary, with a single rotor, instead of the two found in the RX-7 engine.  Simple, compact, lightweight, smooth-running with no reciprocating motion to create vibration, and, most importantly, cheap to build.  It runs at constant speed, driving a generator which in turn charges a battery pack that is integrated into the chassis, adding stiffness and lowering the center-of-gravity of the vehicle.

Each wheel is driven by a combination motor-generator and hydraulic brake assembly.  I’ll call them MGB’s for short.  (Old British sports car enthusiasts note:  This acronym has nothing to do with Morris Garage.)  The MGB drives the wheel and provides regenerative braking for most braking situations.  The hydraulic brake is for “emergency” use when more stopping power is needed, or when regenerative braking cannot be used.  More on that later.

This is a very simple machine!  No complex automatic transmission, drive shaft(s) or differentials are needed.  The MGB’s are connected to the battery via an electronic control unit that regulates the amount of power flowing from the battery to each MGB.  The computer-controlled electronics are well within the current state-of-the-art, similar to those used in current hybrid vehicles.  The controller also starts the engine to charge the batteries when a threshold level of discharge is reached…typically 25%…i.e., when the battery is down to 75% of its capacity.

Okay, here we go.  The controls are simple; just a single switch for Forward or Reverse with a Park position.  There is a rotary switch on the dashboard labeled ENGINE.  It has three positions:  At the top center is AUTO, 90 degrees clockwise is OVERRIDE and 90 degrees counter-clockwise is OFF.  We start out with the switch in AUTO.  In this position, the engine will automatically start when the battery is discharged to the threshold level.  There is one extra gauge on the dash…right next to the fuel gauge is a “charge” gauge.  This indicates the charge level in the battery.

The total battery capacity is sufficient to drive the vehicle around town for about an hour at 30-40 mph.  But after 15 minutes or so, the battery is down to the threshold charge level, and the engine automatically starts…and runs until the battery is back up to 100%.  Then it shuts off.  Meanwhile, driving this vehicle is just like any other…push on the accelerator, and the four MGB’s accelerate the car briskly.  The brake pedal works a bit differently than conventional cars.  Push it gently, and the MGB’s provide nice progressive regenerative braking, which operates the MGB’s as generators, recapturing the kinetic energy of the vehicle, converting it to electricity, and storing it back in the battery.  Push harder, and the hydraulic brakes kick in, stopping you right now!  When we stop at a traffic light, there is complete silence…unless, of course, the engine is recharging the battery.

Around town, mileage is between 80 and 100 miles per gallon, depending on accessory use.  If the A/C and headlights are off on a nice spring or fall day, it should be possible to reach the magic 100.  But today is different.  We are driving from our home in Los Angeles to San Francisco via Interstate 5…up over the Grapevine, a pretty high mountain pass.  Can we do that in this car?  You bet!  It does require a little more driver attention than a 300 hp SUV, and you might be a tad slower on the upgrades, but just think how good it would feel to drive the 450 miles to San Francisco on 5 gallons of diesel!

So, how would it work?  Driving north out of LA, there are a few small grades that would not require any extra attention, but once the Grapevine looms ahead, with its miles of upgrades, we need to prepare for it.  Recall that in AUTO position, the batteries are allowed to discharge to 75% of capacity.  Now, a few miles before we start uphill, we rotate the switch to OVERRIDE.  An orange “Override” light on the dash lights as a reminder.  The engine starts immediately and runs continuously, keeping the battery at 100% charge.  As we start to climb the grade, the engine is no longer able to supply sufficient charge to meet the increased load on the MGB’s, so the battery gradually discharges.  To keep the discharge rate down, we allow the speed to drop to 60 mph, and stay in the right lane.  Monster SUV’s blast past us, engines roaring, but we feel very superior, knowing that they will soon have to stop to fill their 40-gallon tanks.  Oh, I forgot to mention that this trip is taking place a few years in the future, and gas…and diesel…prices are around $10 per gallon.  With the reduced speed and with the engine continuing to pump the charge in, the discharge is slow, and we approach the summit well above the “warning” level  that occurs when we have only 25% charge remaining.  We cannot allow the battery to completely discharge, since it is also used to start the engine.

When we are still a few miles from the summit, we notice that the gauge shows over 50% charge remaining.  We have more than enough charge to make the summit, but now, we are approaching a long downhill, and we want to use regenerative braking as much as possible, so at this point, before we reach the summit, we turn the Engine control switch from OVERRIDE through the AUTO position to the OFF position.  The orange “Override” light goes out and a red “Engine Off” light takes its place.  The engine immediately stops, and we complete the climb to the summit on battery power only.  If we have planned well we will end up with about 25% charge remaining at the top.

Now we start down, and this car picks up speed very rapidly!  There is virtually no driveline friction to slow it.  As the speed rises, we start using regenerative braking to keep the speed down to a legal rate.  We don’t want to get a ticket and blow our trip budget after saving all this money on fuel!  As we continue to apply steady, gentle brake pressure, the MGB units operate as generators, recharging the batteries as they slow the vehicle.  If the downhill is long enough, we may end up completely recharging the batteries before we reach the bottom.  When this happens, the controller stops regenerative braking, and we must rely on the hydraulic brakes, as in a conventional car.  The hydraulic brakes convert the kinetic energy of the vehicle into heat, which is dissipated into the atmosphere and wasted.  We want to use the hydraulic brakes as little as possible.  That’s why we discharged the battery as much as possible before we reached the summit.

Once the battery has recovered to the 75% charge level, or when we reach the bottom of the downgrade, we can return the Engine switch to the AUTO position.  The engine will not start until the MGB units, once again operating as motors driving the wheels, have drawn the battery down to the 75% threshold level.

For the remainder of the trip, through the Great Valley, the terrain is pretty flat.  The engine starts every few minutes to recharge the batteries, and then shuts down.  The sound of the engine is not obtrusive, though.  Running at constant RPM, the intake and exhaust systems utilize active noise-cancellation electronics, and the motor mounts are hydraulic units tuned to damp out the characteristic vibrations of the engine at its constant speed.  We hear only a muted hum, barely discernable over the classical music playing softly on the audio system.

 This car has many other advanced features that make it comfortable and pleasant to drive.  Most of them are available on current production vehicles…such as a closed refrigeration system for the air conditioning (Toyota Prius), adaptive suspension that gives a soft highway ride, but stiffens instantly when evasive maneuvers are required, or twisty roads are encountered.  To minimize wind resistance, airflow is carefully managed…especially on the underside of the vehicle.  This is easier to do than with conventional vehicles, due to the lack of drivetrain components…driveshafts, transmission, differential, etc…protruding into the airstream beneath the vehicle.  Road rumble and wind noise are greatly reduced in the interior with active noise cancellation, using the audio system.  (Acura has such a system in their new RL luxury sedan.)

The only pieces of this vehicle that are not currently available are the rotary diesel engine and the MGB units at each wheel.  I believe the MGB is not a great technological challenge…a motor/generator with integral hydraulic disc brake assembly.  It must be as lightweight as possible, since it is unsprung weight. (See Note 1) The lighter it is, the better the car will handle.  The rotary diesel…well, that’s just an embellishment.  Any small diesel engine would work.  I just like the simplicity and lack of reciprocating motion in the rotary.  There are a few technical hurdles to overcome, but I have confidence that Mazda could do it.  They are a very creative auto company with a lot of experience in rotary engine design.  The batteries need to be light, durable and capable of high charge and discharge rates.  Currently available batteries would work fine, but I would hope for improvements in the future in this critical area.

I think the simplicity of this vehicle will make it very durable and trouble-free.  Another thing I should point out is that the dual brake systems provide a built-in fail-safe redundancy.  This is a very safe, durable, dependable, efficient vehicle…and it shouldn’t cost an arm and a leg either.

So, there you have it…my new hybrid vehicle.  When will they build it?

Did you like the ride?

Note 1:  There is an alternate design approach that mounts the MGB’s directly to the chassis and connects them to the wheels with halfshafts and CV joints.  This would reduce unsprung weight, but increases complexity and cost.  For high performance racing and sporting machines, this might be an acceptable tradeoff, but for a workaday sedan, it would seem to be overkill.

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