Smaller engines and better MPG - discuss.

[obi_waynne]

Hi chaps and chapettes. I had the following email from one of our readers and thought it would make an interesting basis for discussion.

"Don't you think it is fraud to say that a smaller engine will do more mpg than a larger one in the same size car? Apparently, that is a known fact in the motor trade but that is what they put in their brochures "

Here are some thoughts to get you thinking.
If you purchase a car and were sold it on the basis it was the more economical option between 2 models and later you discovered this not to be the case how would you go about resolving this with the dealer/car seller?

Does real world MPG and stated or claimed figures come into this or can you ignore this factor? What about if a car simply doesn't come close to the stated MPG figures - do you have a basis for redress?

 

[HDi fun]

Driving style is the biggest factor in my opinion, journey type the second biggest

 

[pgarner]

think if youve been used to driving a larger engine then when you jump into a smaller engined car youll generally drive it harder.
i know i do anyway

 

[obi_waynne]

More info from our reader:-

"The cars in question were both diesel engines. A 1.6 and a 2.0

I was told that the 1.6 would do better than the 2.0 in the same car but it did not do even as well as a 6 year old 1.9

I tried the 2.0 for a week and it did almost 20 mpg better, my driving my journeys

When I took it back I asked for a like for like journey with their mechanic driving my 1.6 and me the borrowed 2.0

I would go in front to give them the advantage so it would be the same journey, weather conditions etc with an expert driving my car and me, a novice driving the borrowed car. They refused. I know full well that such a test would have demonstrated that the claim that my 1.6 would do better than a 2.0 was not true.

This is my issue. I am not sure that it is clear with what has been put on your forum so I would like to be able to clarify it please.

I have been told that because a small engine has to work harder than a large engine it will not do more mpg than a larger one in the same size car."

 

[thexav]

What are the manufacturers claimed figures for these models? I remember a Prius and BMW M3 on a track and the Prius was driven flat out and returned substantially less MPG than the M3 who could effortlessly keep up!

Manufacturer figures are not really a good guide in my opinion. Perhaps in an automatic more so but driving in the real world is very different from the labs.

 

[Jakeymd]

If you drive a small engined car you have to drive it like a small engined car to get the expected mpg. Like Xavier said, on TopGear the M3 followed the prius round while twidling it's thumbs and the prius was gritting it's teeth. If they had both been driven flat out the prius would have returned better mpg. A small engined car will probably do better in the city with low speeds but a large engine car is much more economical at higher speed.

 

[TCJBOLDIE]

In Australia by law all new cars must have a sticker/label on the windscreen displaying the city and combined fuel consumption figures from a specific test.
I rather think it is a computer controlled test But I will try to locate details and post link .

http://www.greenvehicleguide.gov.au/gvgpublicui/information.aspx?type=fuelconsumptionlabel

Apparently it is carried out on a dyno according to a computerized program

 

[HDi fun]

There seems to be a bit of a trend towards using small engines and multiple turbocharging in a bid to reduce fuel usage. Ford has a 160bhp 1.6 unit which is very very impressive. There's also a 1.0 litre three cylinder unit with 100bhp.

** The 1.6 Ecoboost is now worth 182bhp and the 1.0 litre can be had in 125bhp form ** ! ! ! ! !

BMW's current 528i is actually a 2.0 litre four cylinder turbocharged motor delivery 245bhp.

The logic is simple, smaller engines and fewer cylinders mean less frictional losses.

 

[SLEEPER]

If the engine was a renault then the 1.9 could well be better.

Problem is nothing is ever that simple (as in small is better)

There is always more to it . The size of the car becomes relevant as does the use as it changes the ideal the engine characteristic required.

And that is without going into the relative efficiencies .(a small engine cab give better mpg because it is better design and not because it is smaller)

That is why the garage is wrong if the new car doesnt do as promised - in this instance is more economical
So if it doest give better mpg and it was promised it doest matter why it just matters that it doesn't

A small engine ( diesel or petrol) will be less economical if it struggles to do the job.

 

[TCJBOLDIE]

If you replace say a 2 litre with a 1.6 litre in the same weight body then logically the smaller engine will have to work harder to produce the same on road performance, all things being equal.
There is one inescapable fact you must burn "X" amount of fuel to produce "X" amount of power regardless of motor size.

 

[MasterAuron]

If you replace say a 2 litre with a 1.6 litre in the same weight body then logically the smaller engine will have to work harder to produce the same on road performance, all things being equal.
There is one inescapable fact you must burn "X" amount of fuel to produce "X" amount of power regardless of motor size.

But once you're up to speed given exactly the same gear ratios also which will achieve the greater mpg? Obvs this is where mpg figures come from, and really count imo as I like many others spend most of my driving time at a constant speed, not accelerating like you're talking about.

 

[TCJBOLDIE]

But once you're up to speed given exactly the same gear ratios also which will achieve the greater mpg? Obvs this is where mpg figures come from, and really count imo as I like many others spend most of my driving time at a constant speed, not accelerating like you're talking about.

As a general rule & thinking out loud, logically ,the smaller motor would need a larger throttle opening (IF the TB's were exactly the same diameter)to maintain the same speed provided the level of technology in both was identical.

 

[BMWKid05]

Hi chaps and chapettes. I had the following email from one of our readers and thought it would make an interesting basis for discussion.

"Don't you think it is fraud to say that a smaller engine will do more mpg than a larger one in the same size car? Apparently, that is a known fact in the motor trade but that is what they put in their brochures "

Here are some thoughts to get you thinking.
If you purchase a car and were sold it on the basis it was the more economical option between 2 models and later you discovered this not to be the case how would you go about resolving this with the dealer/car seller?

Does real world MPG and stated or claimed figures come into this or can you ignore this factor? What about if a car simply doesn't come close to the stated MPG figures - do you have a basis for redress?

No its just science on fuel injection