Hybrid Electric Corvette Design Study

What’s the hottest hybrid sports car: The Porsche 918 Spyder, the BMW i8?….and how much do they cost? In the case of the Porsche don’t even ask. You can’t afford it. The i8 is attainable for those that can afford to spend 100,000+ on a novel sports car.

But what about GM? We all know that the current Corvette offers a maximum bang for the buck in the performance department compared to the exotics. Could GM bring this same design philosophy to an electric hybrid version of the Corvette? If they did, what configuration would it be?

A Look at the Competition

It’s always good to know what you are up against. What are the details of the Porsche 918 Spyder and the BMW i8 power trains?

The 918 is an mid engine AWD with an electric front axle. The ICE is a V8 mounted lengthwise ahead of the rear transaxle. Sandwiched between the V8 and the transaxle is an electric motor. The transaxle gearbox is a conventional 7 speed. A battery sits cross wise in the car behind the driver’s seat. The front and rear inverters are separate units instead of integrated ones in the gen 2 Volt.

The i8 is also a mid engine AWD with an electric front axle. The 1.5 liter inline 3 is mounted transversely and mated to the tranaxle assembly resulting in a much more compact design than the Porsche. A belt-driven 20 horsepower motor generator is added in the rear to supply regen and to act as a starter motor. The transaxle gearbox is a conventional 6 speed. The battery runs down the tunnel of the car unlike the 918. The front electric axle incorporates a 2 speed gear box.

GM Design

One thing is clear when looking at the competition. They chose a mid engine AWD design. While some may think that the Corvette is traditionally a front engine RWD car, a mid engine design from GM is not without precedence. The father of the Corvette Zora Duntov was a big fan of mid engine AWD. In fact he designed more than 1 prototype. The most noted being the CERV II shown in figure 3.

Knowing what the competition is doing, what technology could GM leverage to improve on the i8 and 918 designs.

The first and simplest change is to integrate the inverters as GM has done in Gen 2 Volt. The i8 and the 918 use separate inverters.

The second improvement is to use an electric front axle utilizing a co axial gearbox as in the Spark EV. This results in a much more compact design.

The 3rd improvement is in the ICE. BMW is already talking about an M version of the i8 that uses a 4 cylinder instead of a 3 cylinder. GM should therefore just go with a turbo i4 with 2000 cc instead of a 1500 cc turbo 3 boosting power from 228 to 300 horsepower. In addition GM should mount the ICE transversely like the i8 for a more compact transaxle than the Porsche.

The 4th improvement is the biggy. Both i8 and Porsche are using conventional 6 and 7 speed gearboxes. Their transaxles are mostly ICE powered. The main electrical part is the front electric axle. This results in poor performance in EV mode. The i8 0-60 time in EV mode is only around 12 seconds.

GM should leverage Gen 2 Voltec technology in the rear transaxle assembly. This would give the electric corvette superior performance in EV mode over the i8. Existing gen 2 motors could be used. However the mechanicals such as shafts and gears would need to upgraded to handle the higher power output of the ICE.

The performance specs of the new AWD electric Corvette are presented in figure 6.

What is the first thing we notice in the comparison? The new Vette has a respectable 0-60 time in EV mode where the i8 does not. The Corvette has more AER. The new corvette will be a fundrive in EV mode. 4.9 seconds is just fine for everyday electric driving.

The next thing you notice is that the new Corvette blows the doors off the i8 when the ICE is on. The Corvette gets to 60 in 3.2 seconds compared to 4.5 for the i8. This is primarily a result of the whopping 557 combined horsepower when the ICE is on. In EV mode the Corvette still sports 320 horsepower compared to only 129 horsepower for the i8.

I’m sure Zora Duntov would be proud of this new AWD electric Corvette. It will provide superior performance to the i8 for tens of thousands of dollars less.

Source: GM-Volt