What’s an efficient engine? Go figure…

A few years ago, Swedish-Chinese Volvo took the bold decision to stop producing any engine larger than 2 liters and four cylinders, and also to cap the top speed of their cars to 180 km/h (112 mph). At the time I remember thinking they’d lost it, given their largest market was the US and at the time, that was still a market where six or even better, eight cylinders reigned supreme. Oh how the world has changed, oh how wrong I was, and oh how right Volvo was! The brand has continued to grow ever since and the call they made in terms of focusing on smaller, more efficient engines was obviously the right one. But wait – was smaller really more efficient? Since then, we’ve also had various types of hybrids gaining in market share as a more efficient alternative to traditional engines and an alternative to going full electric. Do they really make sense, and if yes, for whom? There are indeed a lot of questions around this topic which is more complex than you may think, so this week I’ll try to answer at least some of them. As a small hint, efficiency is not always what it is portrayed to be…

Ok, it wasn’t with the 240, but it was still a return to its roots Volvo did

Long-term readers may remember I used to have an XC90 T6 that I bought by trading in my AMG E63 (I know, I know, even my wife bashes me for that). It’ was never a car I came to love for various reasons, the 2-liter, four-cylinder, double-turbo engine being one of them. It produced 310 hp in standard but mine had had the mild Polestar chipping increasing power by 20 hp, and also said to improve fuel efficiency. It was fine and very discreet on longer trips, but on shorter drives you would constantly be reminded of it being turbo-powered, far from the far less stressed character of a larger engine. No wonder given the car weighed close to 2.3 tonnes which is a lot for such a small engine. The turbos came in pretty constantly on longer distances as well even if you don’t notice it, meaning the care on average consumed 11.9l / 100 km (around 20 MPG US), which isn’t terrible but certainly not sensational either.

I traded in (or rather sold) the XC90 for my current Range Rover with the 5-liter, supercharged V8 which I’ve since driven around 15.000 km in a similar mix to the Volvo and at similar “almost legal” speeds between Zurich, the mountains and southern Europe. I’ve so far had an average consumption of 12.8 liters, i.e. less than a liter more than the XC90. How can that be given a larger engine by definition needs more fuel? Well, quite simply because the larger engine doesn’t need to be forced to the same degree as a smaller one. Less stress mens lower rpm and usually also more longevity. It goes without saying that the V8 character of the Range Rover is also a far more pleasant experience. Of course this is too much for you average politician to take in, so it’s not uncommon in different countries for larger volume engines to be taxed heavier based on their volume and not their consumption.

O how much more pleasant a V8 is to a 2-liter, 4-cylinder…

The above is important to remember if you’re considering buying one of the modern four cylinders, where it’s not uncommon to get 300 hp+ out of a cylinder volume of 2 liters. The price for doing so is constantly engaging turbos, so if you drive your car in the sporty manner it invites you to, your consumption will be nowhere near what’s quoted in the prospectus.

Another thing to get your heard around are the new hybrid engines that combine a petrol or diesel engine with an electric one. These work along two principles:

  • a mild hybrid is the smaller variant. it’s not charged over a cable but rather by the regenerative energy from the normal engine and from braking. A mild hybrid will support the normal engine under certain driving conditions such as to lower the average consumption by something like 5-10% depending on model. Next to the lower consumption, its advantage is the small size of the battery (typically below or around 10 KwH), meaning it doesn’t add much weight or takes a lot of room.
  • A plug-in hybrid has a larger battery, these days typically between 10-20 kwH. These are charged over a cable like an EV and they allow the car to be driven in electrical mode for a number of km which will depend on the size of the battery and the type of driving, but where new plug-ins manage up to something like 80 km. Once the battery empty, the car switches back to the normal engine and the battery then needs to be recharged over a cable – it’s not recharged by the car itself.

The above means that how and where you drive is decisive for what type of hybrid you should get, if at all. As a rule of thumb, if over half of your driving is shorter distances, then a plug-in hybrid makes sense as you’ll be able to do almost all of these trips in electric mode. If on the other hand you mostly drive longer distances, a mild hybrid will be far more efficient as it will work over the full distance rather than only for a small part and only add significant weight thereafter. The big question is of course why no one builds a plug-in hybrid that works like a mild hybrid does, and that’s a question I can’t answer. It would certainly be possible from a technical standpoint, and that’s why I think that pretty soon when it becomes clear that we neither have enough energy nor resources for global electrification, the solution could well be a small, 2-3 cylinder engine combined with battery power. We’re starting to see such concepts here and there, and we will most probably see more going forward.

In summary therefore, a small engine will consume significantly less fuel only if it’s associated to a small/light car and driven in a civilized manner. That however means that if you’re buying a small car for its economy, you don’t need the double-turbo super version of that engine but can settle for a smaller version. If you drive a lot of shorter distances, then a plug-in may be a sensible choice. If however you drive a mix of shorter and longer distances as most of us do, then the answer is not any form of hybrid as in that mix, nothing is more efficient than a modern diesel engine. It wasn’t the case in the old days but modern diesels are as clean as petrol cars and they bring unrivaled advantages in torque and consumption. A modern six-cylinder diesel with 250-300 hp will have 600-700 Nm of torque, i.e. all the power you need, and still keep you below 10l/100km (above 23.5 mpg US) even in a larger car, and around less than half in a smaller car. Thus, ff you’re concerned about future fuel prices (a very valid concern) and about the downsides of going fully electric (obviously even more valid), then a modern diesel is in many cases the way to go!

One thought on “What’s an efficient engine? Go figure…

  1. Pingback: The best power SUV in the market is a bargain! – The Thrill of Driving

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