The Ingenuity of Two Engines

 

Hybrids are the way forward. I’ve said it for years. And plug-in hybrid electric vehicles (PHEV) are the type I particularly advocate for. And these should be diesel with a rear electric motor. Hear me out.

For the uninitiated, a PHEV uses a combustion engine of whatever size—usually as small as 1.5 litres to as large as 4.4 litres—along with an electric motor and battery pack. This PHEV battery pack is much smaller than the one in a full or battery electric vehicle (BEV) but larger than that from a hybrid electric vehicle (HEV). And, like a BEV or simply EV, it can be plugged in; hence the name. I’m going to go into more details to make for an elaborate explanation.

For instance, an EV may have a battery pack of as little as 17kWh (MG Comet) or over 100kWh (any luxury EV) depending on the size and type of vehicle. Smaller packs are lighter. Larger capacity packs are heavier. Comparatively, a hybrid (HEV) uses a battery pack usually of 1.5kWh give or take (Kia Sorento 1.49kWh, Toyota Camry 1.75kWh). This means it works on a quick discharge-recharge cycle using the vehicle’s momentum, engine and brakes to recuperate energy.

There are also mild-hybrid electric vehicles (MHEV) on 12- or 48-volt systems with even smaller battery packs (350Wh or 0.35kWh Maserati models, 960Wh BMW models) but most of them are merely equipped to handle start/stop and mild torque-assist functions via a belt alternator-starter-generator. But those are mostly stop-gap measures meant to sneak past emission norms, in my opinion. Lately, Audi “MHEV plus” models are offered with a 1.75kWh 48-volt battery pack and a stronger electric motor (17kW or 23hp and 200 Nm) capable of driving the vehicle or extending coasting functionality for much longer. That makes more sense to me, but on to the essence of this piece.

A PHEV uses a larger battery pack to provide electric drive range for longer distances and with more power. It can have anywhere from 6kWh (Toyota Prius+) or 30+kWh (Mercedes e-suffix models like the C 300 e or S 450 e) to give a fair amount of range; this could be anywhere between 40 to 100 km or more without any need for the engine kicking in. Then there is the motor placement itself—and we’re getting to the original point here. Many PHEV models have an engine and e-motor driving only the front wheels in the interest of efficiency. Many have rear- or all-wheel drive and have the electric motor sandwiched between the engine and transmission—specifically between the engineoutput shaft and transmission input shaft—so that the driveline works with or without the engine on, with a simple lock-up clutch handling seamless transition between modes. And then, there’s another kind. And it’s the early BMW X2 xDrive25e which is of particular interest.

 

The engine is in the front and drives the front wheels. The electric motor is on the rear axle and drives the rear wheels. There is no prop-shaft or transaxle—saving crucial weight—with just the wire sending power, and the battery doesn’t need to be very large either. This setup enables four-wheel drive with combustion front and on-demand electric rear, or just pure-electric rear-wheel drive!

The X2 wasn’t the first to do this. Three Volvo T8 models (S90, XC90 and then XC60) used this set-up as well, and so did the BMW i8 sports car. All four of these used an electric motor on the rear axle with a two-speed transmission to optimise power delivery and heat management. Toyota and Lexus HEV e-Four models also adopt this approach for on-demand electric rear-wheel drive. It’s lighter, more software-hardware-dependent with lesser mechanical losses to deal with. The most efficient way to go about it? I think so. What do you think?

 

Honestly, those are all petrols. The Volvo V60 D6 PHEV AWD was the real star of its time for me. It combined a diesel engine at the front with a rear electric motor and had three modes: Pure (for full EV drive), Hybrid (using EV drive but with the engine kicking in as a generator) and Power (both driving their respective set of wheels).

 

I still believe diesel plug-in hybrids make the most sense. Diesel engines deliver the necessary torque far lower down the engine speed range than petrols. A diesel can do at 1,500 rpm what a petrol needs 4,000 rpm or more to do, in most cases. The inherent torque-rich nature from higher compression also helps with efficiency.

If we are to save the planet, a small hatchback (sub-4m) with a 1.5-litre turbo-diesel four-cylinder engine (four for a better balance and weighty crank—like Toyota’s excellent 1.4-litre D-4D in the Etios/Corolla), paired with a 20-30kW electric motor on the rear axle, with a 3-6kWh battery pack will easily deliver exceptional efficiency and outstanding driving range with minimal emissions.

  

Think about it , if a Mercedes C 300 de plug-in hybrid diesel can be rated at 0.4L/100km (250 km/litre) on the WLTP low test cycle, why not a split-drive little diesel-electric hatchback weighing nearly half as much. I’d love to see someone like a Hyundai take an i20 or a Venue 1.5 CRDI diesel and pop a 20kW motor and 3kWh battery from an electric motorcycle at the rear along with the necessary plumbing and drive management system calibration. Even a small car will not come cheap, of course. But we can’t put a price on the environment, can we?

 

The way it’s going right now, the focus is on making cheap and efficient mass-market cars for people to “upgrade” to. The angle is profit, not purely environmental. And that also makes me question the “Cleaner EV” narrative. Isn’t a two-tonne medium-sized EV the furthest thing from ideal?

Note: one kWh = one unit of electricity. One single charge of one 110-kWh battery pack of one car is as much electricity or even more than what I need for my 1BHK flat for a whole non-summer month. Do the math!