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Beating the Prius at its own game

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Building the Ultimate Hybrid

Let’s face it, Hybrids are not about sensibilities as much as CTS-Vs are not about sensibilities. A sensible buyer will understand that a Prius does not save you any money over a Corolla and a CTS 3.6 gets you around with better comfort while burning less gas than it’s V brethren. Hybrids are about making the owners feel good. And, while CTS-V owners feel good about getting pressed to their seats, being able to carve mountain roads at highly illegal speeds should they choose to do so and secretly musing about the parody of driving to the supermarket in a 556hp rocket sled in sheep’s clothing, Hybrid owners enjoy watching the MPG gauge return ridiculously high numbers and brag about the efficiency of their powertrains in front of their Global Warming coolaid drinking buddies while patting each other on the back.

The Problem with GM’s Hybrid strategy is that it is too sensible and insufficiently sensational. The Mild Hybrids and the Dual Mode SUVs are make the most economic sense. The Volt strikes a balance between zero emissions of electrics and the practicality of being able to take long trips and drive without worry of being stranded. But BAS cars aren’t posting Prius numbers and are in fact bottom of the class in MPG bragging rights. The Volt, well, it’s not spectacular looking, it’s not fast, it’s electric range is a paltry 40 miles and it’s hybrid mode highway MPG is no better than a Cruze or Focus. In short, it’s not a Model S. In a way, I guess one can say the same of Honda’s strategy – it’s too sensible.

What GM really needs is a no excuse Hybrid that blows away it’s competition in the MPG game in its price class. As far as price class goes, $30K is not bad place to be. It’s $4K more than a Prius but it’s cheap enough to draw the Prius buyers if they really want one. What’s needed is substantially better MPG numbers than the Prius. And, that is actually a walk in the park from a technical standpoint with $10K to play with over the $20K price tag of a loaded Cruze LTZ. Here’s how I’ll do it…

Chevy Slipstream

Platform: Aluminized Delta / 4-door

Sitting arrangement: 2+3

Engine: 1.6L Naturally Aspirated Diesel -- 70 hp @ 5200 rpm / 110 lb-ft @ 2600 rpm (Opel 1.6 CDTI Ecotec Diesel w/o the turbo)

Motor-Generator: Permanent Magnet DC motor -- 70 hp @ 5200 rpm / 141 lb-ft @ 0 rpm

Combined output: 140 hp @ 5200 rpm / 216 lb-ft @ 2600 rpm

Auxiliary Generator: 15kWe Exhaust Gas Turbine Generator

Transmission: 9-speed Automatic without torque converter or clutch

Battery: 5.5 kWh, 360 V Lithium-Manganese-Oxide battery w/ 96 cells (basically one of the three parallel 96-cell modules used in the Volt)

Target MPG: 70 mpg City / 70 mpg highway (w/o plug-in usage)

The Powertrain concept is very simple really. Instead of a gasoline engine we use a diesel which is immediately about 20% more efficient at the same output. But we are not going to just stop there. We’ll use the smallest diesel GM has and make it naturally aspirated. The reason for dropping the turbocharger two fold – first it allows for higher static compression which improves cruise and low output fuel economy which is what we need for those EPA test cycles, second it allows all that exhaust gases to be used for something else. That something else is the hallmark of this hybrid drive train – a second source for energy to charge the battery apart from regenerative braking. Instead of using the otherwise wasted exhaust to increase the specific output of the engine through turbo charging, we recover it as electricity via an exhaust driven generator. Any additional power for the wheels come from the electric motor if and when warranted.

The transmission system is an utterly conventional 9-speed Automatic currently under development with one catch. It doesn’t have a torque converter or an automated clutch. Instead, the transmission input is coupled directly to the engine’s crankshaft and the 70hp electric motor-generator -- all three turn at the same speed or not at all. Basically, this engine is incapable of idling. At about 800 rpm, the VVL system basically shuts off all the valves and allow the engine to freewheel without pumping losses. Below 800 rpm all propulsion is electric. The motor spins the transmission as well as the freewheeling engine. Above 800 rpm, fuel is injected into the engine and the valves start working again reactivating the engine. During all deceleration the VVL system again deactivates the engine eliminating engine braking such that all of the powertrain braking is regenerative. A torque converter is no longer needed because the engine does not need to idle and during shifts the electric motor pitches in with a little torque input or regenerative outtake to unload the transmission – basically rev and load matching such that the gears are not experiencing significant accelerative or decelerative loading. A 5.5kW battery with about 4kW actually used provide the assurance that there will be plenty of juice for puttering around under 800 rpm in 1st gear. The car has a 12~15 mile all electric range and is capable of an emergency recovery mode in the extremely unlikely circumstance that charges are drained below the minimum allowed on the battery (about 15% charge). If a the battery gauge ever reaches the regulated “0”, the car can run the engine in neutral to bring it back up to drive away levels and the car will never leave the driver stranded.

At full throttle the system sends all available engine and electric motor power to the wheels. But at part throttle it is smart about it. When the battery is above 33% it uses more electric power before increasing ICE power. Below that, it moderates electrical assist at part loads to build the bias towards charge recovery and storage. An optional SAE 1772 receptacle allows those with patience for the hassle to recharge the battery via a wall 110v socket in 10 hours or a 220v in about 4 hours. The charging kit is exactly the same as the Volt’s. If the owner doesn’t plug it in the vehicle is designed to function like a traditional Hybrid recovering which decelerative energy from regenerative braking, as well as slowly build up the battery charge from the a the gas turbine generator driven by the diesel engine’s exhaust.

The diesel engine adds about $1000 to the cost. The gas turbine generator is basically the exhaust half of a turbo coupled to an alternator and is about the same price as the 1.4T’s turbo, intercooler and pipings. The battery is a good $4K (slightly more than 1/3 the volts battery price). The 74 kW electric motor is about $2500. There is no inverter and expensive multi-phase power controller since we are going to a DC motor. The torque converter-less transmission is actually cheaper than it’s traditional equivalent. That’s about a $7500 premium in costs, making a $30K car perfectly doable. The logic here is that we are using the money saved from eliminating the Volt’s battery, 149 hp motor to put together a conventional hybrid with that beats the Prius at its own game with a $30K vehicle (the originally intended price point of the Volt).

This setup is more efficient than the Prius's because:-

  • It uses a Diesel ICE
  • It has two energy sources for the Hybrid system -- regenerative braking + exhaust gas turbine generation
  • It has complete cylinder deactivation allow full regenerative braking using the motor
  • It has a much higher battery capacity
  • It has a multi-speed transmission instead of a planetary power spliting device.
Edited by dwightlooi
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Wouldn't that engine shut down technique be rather jerky... particularly in a diesel? I mean, if I can feel the A/C compressor kicking on and off through the gas pedal on lower powered cars, the drag of spinning a dead diesel is going to be felt too. Also, diesel engines don't really like being shut off and then back on again often. It's bad for their health, and they are noisy and jerky about it.

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Not really... The reason cranking an engine is "jerky" is that the effort is not constant. That is the engine is compression four cylinders in succession and each compression stroke takes more effort towards the end than in the beginning. If you have deactivated the engine by closing all the valves, the reciprocating assembly is now a balanced spring system. Any effort compressing one cylinder is balanced by pressure assisting the downward stroke on another. Apart from the frictional drag there is no effort and the frictional drag is constant at any given rpm.

You'll reactivate the engine by injecting fuel into the cylinder in its compression stroke. Near TDC the combination of heat and pressure will light the charge. To keep cylinder temperatures in you'll probably close the thermostat and keep the coolant hot. If the engine has been off more a few minutes and it's cold outside you may turn on the glow plugs. To moderate the "jerkiness" when the cylinders first light up you'll not inject the amount of fuel called for by the current throttle position but rather ramp it up for the minimum. The first injection pulse gives only enough power to overcome the drag at 800 rpm so that first ignition is essentially imperceptible. Power ramps up in the next dozen or so injections over a ~1/2 second period of time. It is not unlike idle start with a gasoline engined hybrid like the Prius except you don't have that planetary power spliter and you don't have a spark.

If you are thinking about the dieseling effect after an engine is shut off, that essentially cease to exist when the engine is shutoff by shutting all its valves. Modern diesels use a shut off flap to cut air supply and eliminate dieselling which is what happened when you cut off the fuel to a compression ignition engine that doesn't have a throttle body. Shutting all the valves has the same effect, if fact it is more immediate and more effective because there is essentially zero reserve air volume.

Edited by dwightlooi
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Because diesel being a compression ignition engine as opposed to a spark ignition engine and operating at with no throttle body constrictions all the time has better thermal efficiency than CNG. Diesel is more available than CNG and a given volume of diesel or a given weight (inlcuding the tank) of diesel goes further than an equivalent amount of contained mass of CNG.

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I understand that you would eliminate pumping losses, but there is still a large change in overall effort there.

And why not just use Start-Stop at idle instead of using the electric motor to keep the diesel engine spinning?

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I understand that you would eliminate pumping losses, but there is still a large change in overall effort there.

And why not just use Start-Stop at idle instead of using the electric motor to keep the diesel engine spinning?

Two reasons...

(1) The whole idea is that you want to get rid of the torque lossy converter or complex electrohydraulic clutch assembly. This way, the engine, motor and transmission input shaft spins together all the time with no slippage losses.

(2) It forces the ICE to turn over whenever the car is moving. And, not only that but turn over at the same speed as the electric motor. This eliminates the need for separate starter or the need for a separate cranking event to start the engine. The lack of a speed difference eliminates the need to decouple the engine from the motor or the transmission. There is no deliberate starting of the engine, the moment electric motor rpm goes from 0 to 0.00001 the engine starts turning. The only thing that happens at ~800 rpms is that instead of being a pure load, the engine starts to make power. As far as being a load, the effort is akinned to reving an engine with the heads removed (no pumping loss). Yes, it's there, but it is relatively insignificant compared to the load of actually turning wheels that must push 3000 lbs of car around. An analogy will be the power used to run the turn-table in the microwave oven as opposed to that used to generate the food heating microave radiation itself.

Edited by dwightlooi
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