Are there any reasons why I should avoid cars with turbo 3 cylinder engines?

Idomeneus

In the last few years, an increasing number of cars have appeared on the market with turbocharged 3 cylinder engines, particularly in the supermini class which I am currently interested in.

Now, I’m familiar with the reasons why this is happening - smaller capacity engines with more power than larger capacity 4 cylinder engines, and so on. These reasons are well documented by the car manufacturers. I’m also familiar with the traditional argument that 3 cylinder engines don’t run as smoothly as 4 cylinder engines, and I understand the technical reason why.

The other day, I had a test drive in a new Peugeot 208. It was only when I got home and started to read the specification of the Peugeot 208 did I realise that I had been driving a turbo 3 cylinder engine.

Besides the technical argument against 3 cylinder engines referred to above, does anyone know of any practical reasons why turbo 3 cylinder engines should be avoided? For example, do they tend to exhibit problems that 4 cylinder engines don’t exhibit? Do they tend to wear quicker and don’t last as long? And so on.

During my test drive in the Peugeot 208, I noticed one thing that concerned me a little. When travelling at a constant speed of 60-70 mph on a motorway, the rpm dial displayed about 2500 rpm, which I was expecting. However, when travelling at a constant speed of about 30 mph, the dial was also displaying about 2500 rpm. I would normally have expected the dial to display about 1500 rpm under these circumstances. Is there any theoretical reason why a 3 cylinder engine requires more rpm than a 4 cylinder engine to propel a car at a given constant speed?

Of course, this anomaly could be explained by the behaviour of the automatic gearbox in the Peugeot 208 that I was driving. I’m concerned because I usually equate higher rpm with higher fuel consumption.

AdrianC

Idomeneus wrote: »

Besides the technical argument against 3 cylinder engines referred to above, does anyone know of any practical reasons why turbo 3 cylinder engines should be avoided? For example, do they tend to exhibit problems that 4 cylinder engines don’t exhibit? Do they tend to wear quicker and don’t last as long? And so on.

It's not so much the number of cylinders. It's more a question of small-displacement, high-relative-power engines being more heavily stressed than larger, lower-relative-power ones. In the long term, there probably will be reliability issues. In the short-to-medium term, those are outweighed by higher efficiency, so lower fuel consumption and CO2.

During my test drive in the Peugeot 208, I noticed one thing that concerned me a little. When travelling at a constant speed of 60-70 mph on a motorway, the rpm dial displayed about 2500 rpm, which I was expecting. However, when travelling at a constant speed of about 30 mph, the dial was also displaying about 2500 rpm. I would normally have expected the dial to display about 1500 rpm under these circumstances. Is there any theoretical reason why a 3 cylinder engine requires more rpm than a 4 cylinder engine to propel a car at a given constant speed?

The number of cylinders is irrelevant. A certain amount of power is required for a given speed, but the amount of power developed at any given engine speed is entirely dependent on the throttle position. Too much power to hold a given speed, the car accelerates. Too little, it decelerates. The relationship between revs and road speed is determined by the gear the car's in.

Of course, this anomaly could be explained by the behaviour of the automatic gearbox in the Peugeot 208 that I was driving.

Well, yes. I'm presuming you've not driven many automatics?

I’m concerned because I usually equate higher rpm with higher fuel consumption.

Nowhere near that simple. Any even half-way modern automatic will ensure it's in the most efficient gear at any given time.

Ebe_Scrooge

There has always been a trend in Europe for smaller, but higher-stressed engines. The smaller engine gives better fuel economy and lower emissions, but it has to work much harder to give a decent power output ( as evidenced by your example where it needs a turbo ). Short-term this won't be a problem, but it will have a shorter lifespan than a larger-engine car of similar power output.


The best example of this is in America ( well, certainly up until recent times ). Fuel there was always very cheap, so the trend was to have very large-engine cars ( e.g. 5 or 6 litres, or even more ). Even when travelling fast the engine was barely ticking over, so it tended to last a very long time. Of course, an engine of that size guzzles the fuel, but if fuel is dirt cheap then it's not really a problem.

iolanthe07

Even when travelling fast the engine was barely ticking over, so it tended to last a very long time.

When I lived in the USA I had a 5.0 litre Pontiac V8 which showed 1500 rpm at 60 mph. These cars just went on for ever - no corrosion in California - and there were examples with 500K+ miles and no major surgery required. I wonder how long these little turbo three cylinder engines will last; they must be highly stressed. Perhaps with modern metallurgy and lubricants they'll be OK. Time will tell.

Idomeneus

AdrianC wrote: »

... Too much power to hold a given speed, the car accelerates. Too little, it decelerates. The relationship between revs and road speed is determined by the gear the car's in.

... Well, yes. I'm presuming you've not driven many automatics?

... Nowhere near that simple. Any even half-way modern automatic will ensure it's in the most efficient gear at any given time.

My thanks to you and the others for the replies. There is certainly a common theme that runs through all three replies and I shall bear it in mind as I look for my next car.

I didn't quite understand what you were saying in the first paragraph of the quote above. If a car is travelling at constant speed, it is neither accelerating nor decelerating.

Actually, I've been driving automatics for at least 20 years. I'm completely sold on them.

I agree with your final comment. When travelling at a constant speed of 30mph in my Honda Jazz with a CVT automatic gearbox, the rpm dial displays about 1500rpm. That is why I was surprised when, on the Peugeot 208 with an automatic gearbox, the rpm dial displayed about 2500rpm.

bigjl

Do you understand the difference between a CVT auto and the Autobox fitted to the 208, which Inwould suspect is probably the usual PSA automated manual.

Why would the revs be different at 30mph and 60/70mph?

The gearbox would simply be in a different gear, which is what AdrianC was getting at when he said the box would select the most efficient gear.

Which at 30mph might be 3rd or 4th and at 60/70mph might be 5th or even 6th if the box has 6gears.

A CVT hasn't really got any "gears" there might be set positions that the engine may default to, but the actual ratios will be constantly variable.

Hence why if you hit the throttle pedal for max acceleration it simply revs to a higher amount and maintains it with the box altering the ratios as road speed increases. If you lift off in a CVT the revs will drop quickly.

I personally don't like driving CVT boxes that much as they are pretty noisy.

The Jazz is also one of the worst Autos on the market mainly due to the CVT box.

AdrianC

Idomeneus wrote: »

I didn't quite understand what you were saying in the first paragraph of the quote above. If a car is travelling at constant speed, it is neither accelerating nor decelerating.

Exactly.

Travelling at a certain speed - balanced throttle, the car neither accelerating or decelerating - the amount of power being generated at that engine speed and that throttle position is the amount of power required to overcome friction and aerodynamic drag. Increase the power, by increasing the throttle position, the car accelerates. Decrease the power, by decreasing the throttle position, the car decelerates.

I agree with your final comment. When travelling at a constant speed of 30mph in my Honda Jazz with a CVT automatic gearbox, the rpm dial displays about 1500rpm. That is why I was surprised when, on the Peugeot 208 with an automatic gearbox, the rpm dial displayed about 2500rpm.

A CVT is a different kettle of fish. A CVT will vary the ratio infinitely - hence Constantly Variable Transmission - whereas an auto has a fixed number of ratios.

Richard53

Idomeneus wrote: »

I’m also familiar with the traditional argument that 3 cylinder engines don’t run as smoothly as 4 cylinder engines, and I understand the technical reason why.

A 3-cylinder inline engine can be built to have perfect primary balance. Unless you go for a V configuration (or an inline 6 or 8) or add things like balance shafts, it's the smoothest there is in terms of vibration. However, for any given engine speed, the power pulses are further apart, which can lead to the power delivery feeling slightly rougher than a four. This is desirable in a motorcycle as it aids rear wheel traction, but perhaps less so in a car. However, there is no technical reason why a 3-cylinder car can't be as smooth as you like.


I'm not an engineer, but that's my understanding. If anyone knows better and can educate me further, I'd be interested.

WellKnownSid

Richard53 wrote: »

A 3-cylinder inline engine can be built to have perfect primary balance.

Rotationally they can be balanced, but they cannot do anything but fire in order - either one way or the other - which means they rock front to back.

londonTiger

drove an up on zipcar it drove fine, actually was quite perky, i kept speeding in it. the 3 cylinder may not be powerful enough to pull a caravan, but it will be fine for the small cars they're placed in

Richard53

WellKnownSid wrote: »

Rotationally they can be balanced, but they cannot do anything but fire in order - either one way or the other - which means they rock front to back.

Which is why I said 'primary balance'


Triples work best with a short crank for this very reason, and perhaps at a larger capacity in a car it may not be ideal. My Triumph Sprint was the smoothest bike I had that wasn't a V engine.