The Doctor

WHERE ARE THE DIESEL PLUG-IN HYBRIDS?

Hi Doc, I recently spent a few days with my family in northeast Poland, almost adjacent to the crazy military border with Belarus. We stayed in a cabin which has nothing; no running water, no gas and no electricity. This gave us lots of time to talk and play. In the parking lot were two diesel SUVs and a diesel estate. One discussion we had was what cars would we buy to replace our existing transport. The continuous use of EVs was immediately eliminated. The nearest fuel stations are some 30km away from our cabin and nobody had any idea where electric chargers are located. But we all recognised the environmental benefits of using such cars in cities. But to carry out the long drives we all had done to get to our cabin, again we all agreed this time that diesel is best, even some motoring magazines with journalists who have no bias agree with us. None of us could understand why so many manufacturers have produced hybrid cars comparable to Toyota technology of 20 years ago, using the timescales from Diesel Car magazine, that give no advantage today when compared to average modern diesel engines. So our conclusion was to evaluate the availability of plug-in diesel hybrids.

Using Diesel Car magazine, we could only find one rather expensive Mercedes-Benz, not something we'd ever considered previously. So our first question is why are there so few plugin diesel hybrids available, especially as logically this would deliver the best driving experience possible today, if you can only afford one car? During the heyday of diesel cars, reports were produced that documented the carbon savings this had generated, are there similar reports available today that document the increase in global warming carbon due to the switch back to petrol and ridiculous hybrids with massive petrol engines, we could only assume these huge capacity petrol engines are required to provide the same performance as a modestly sized diesel? And as manufacturers can face penalties for increases in exhaust pollution, how many have been accused of this so far? Today, I don't easily find the pollution comparisons of different car exhausts, why is that? Unless you have any answers that changes our minds, we have decided to stay with the diesel cars which we currently have for the long term, unless manufacturers change the directions they are taking from stupid hybrid, to plug in petrol, to EVs. Maybe in the years to come we'll be able to find another propulsion alternative, which we have discussed several times previously, and which at least is being considered more urgently than it was.

It can't be easy at Diesel Car magazine with so few new diesels being introduced. As a long-term reader, keep going as long as you can. Take good care of yourself. Bill

Hello Bill, Always good to hear from you. As you rightly say, diesel plug-in hybrids are extremely rare. Right now, you’re limited to a new Mercedes-Benz, but previously you could have bought one from Audi or Volvo. Going back further, you might recall PSA’s HYbrid4 technology, which debuted in the Peugeot 3008. There are differences. In a new Mercedes-Benz, the diesel engine and electric motor work in harmony, while the old French tech sees them at different ends of the car.

There are many reasons why the industry hasn’t embraced diesel-electric technology. If nothing else, the antidiesel agenda means it’s hard to mount a case for a diesel hybrid, even if the range, economy and emissions figures stack up. The Mercedes-Benz E 300 de is a phenomenal car. Consider the stats: 33 miles of electric range, 11% BiK, 217.3mpg and 0-62mph in 5.9 seconds. The problem is, it’s not cheap. You’re looking at the best part of £50k before you drive away from a showroom, although I accept that most owners will be leasing or running one as a company car. However, it raises the wider point that diesel engines and electric motors are expensive. Put the two together and you create something that only makes sense if the car buyer can justify the investment. There’s little point developing a diesel plug-in hybrid for a family hatchback or compact crossover. The figures just don’t add up.

Then there’s the fact that petrol cars require the additional fuel economy advantages more than a diesel engine. Petrol engines are less efficient than diesel engines, with less fuel converted into forward motion. That, and the fact that petrol engines and electric motors make good partners; electric is great at low speeds while petrol peaks as the revs rise. Diesel engines and electric motors aren’t quite as harmonious.

The sun will soon set on the diesel plug-in hybrid. Manufacturers are investing in battery electric vehicles, hydrogen and so-called ‘self-charging’ petrol hybrids, so the small selection of diesel plug-ins is set to get even smaller.

The slow decline of the diesel engine shouldn’t have too much impact on the magazine, which has been quick to embrace the new breed of eco vehicles. Our captain (editor) has done a fine job of steering the ship through the stormy waters, maintaining its authority and credibility. I just hope he remembers to stick the cheque in the post in recognition of these positive words.

All the best, Bill, and stay safe. Doc

H O W I T W RKS

FWD, RWD and AWD in EVs

For most small and medium hatchbacks and SUVs, frontwheel drive (FWD) has long been the norm, yet many new dedicated allelectric cars are seemingly reverting back to the classic rear-drive layout. New EV models created as purely electric cars from the outset, like the BMW i3, the Tesla Model 3, the Hyundai Ioniq 5, and Kia EV6, and of course the new Volkswagen ID. range of EVs, have all opted to place the power unit at the rear, driving the rear wheels. Tesla play the tunes with powerplant positioning, and drive, using RWD and AWD, but no Teslas use FWD. But then there’s the Nissan Ariya, and its recently revealed cousin, the Renault Megane E-Tech, and Mazda with the MX-30 have chosen the FWD route, with a front-located power unit. Why the split-decision on such key design principles? Surely one must be better than the other?

Well, with few exceptions, like the Hillman Imp, the Beetle, and the more recent Renault Twingo, most power units have been located at the front, leaving designers a choice of driving the front wheels, or using a prop shaft to drive the rear wheels. Both RWD and FWD layouts have both critics and supporters, but it's really a case of fitness for purpose. Driven front wheels save space, and avoid long prop shafts, but have to cope with the tasks of both steering and power delivery, compromising both functions. For such reasons prestige car companies like BMW, Mercedes-Benz, and Jaguar stuck to RWD for their larger models, where cabin space was no great issue. But many RWD cars had a tendency to be tail-happy on slippery surfaces, and even lose rear end grip when too much power was fed to a lightly loaded rear axle. BMW compensated for this by achieving near 50/50 front/ rear weight distribution, placing the engine as far behind the front axle as possible, and locating the heavy battery beneath the boot floor. They felt that the purity of tactile steering, uncorrupted by drive power, and the superior handling of 50/50 weight distribution justified the loss of some space. The arrival of Electronic Stability Control technology in the early 90s, that tamed most RWD car grip shortcomings, further strengthened their argument.

But electric motors are physically much smaller and lighter than petrol and diesel engines, allowing them to be located at the rear, often without seriously compromising the cargo space, and allowing RWD configuration, with a near ideal weight distribution. The compactness of the electric powertrain has simply changed the game. Gone is the huge weight of a petrol or diesel engine, clutch, and gearbox, replaced by an electric motor, its power electronics and charging electrics, together weighing less than a third of the heaviest single EV component, the traction battery. This is located low down, usually beneath the central and rear cabin floor, creating a low centre of gravity, along with a low “moment of inertia,” which together improve vehicle stability. It leaves space for the main electrical drive parts to fit comfortably between the rear wheels, whilst the removal of rigid, and costly powertrain components to the rear increases the effectiveness of front crumple zones and the ability to absorb energy in frontal impacts. Arguably it's the best of both worlds, with the power unit directly over the driven wheels, and it offers the potential to locate a second power unit at the front creating a 4WD variant, as many have done. This delivers the combined benefits of almost perfect weight distribution with the electronically controlled power delivery spread over four wheels, for optimum traction at all times. Some manufacturers also add a mild touch of rear wheel steering, where the steered wheels turn one way at low speeds, improving the turning circle of larger models, and turn the opposite way for improved stability at high speeds.

The evolution of the EV is certainly at an interesting stage, and time will tell which way things go. Our betting would be that all the next generations of larger “pure” EVs will generally choose RWD, often with a 4WD option, whilst smaller pure EVs, when they arrive in larger numbers, as surely they must, may well take the opposite route of FWD, for the traditional reasons of maximising cabin and cargo space. Then again, we have the revolutionary possibility of direct drive, motor-inwheel technology, of which we shall inevitably hear more about in time…

“...MOST POWER UNITS HAVE BEEN LOCATED AT THE FRONT, LEAVING DESIGNERS A CHOICE OF DRIVING THE FRONT WHEELS, OR USING A PROP SHAFT TO DRIVE THE REAR WHEELS.”