Why do 4x4s need/have big engines...?

[Lorin]

Alright, this is probably a thick question but I'd be interested to know the answer rather than just guessing

Why do the bigger 4x4s need/have large capacity engines At a guess I am assuming it is simply because they are physically stronger and no other reason - am I right?

The reason I ask is that there are now a lot of small capacity 4 cyclinder diesels that produce more power than the heavy old lump in an 80 for significantly less fuel consumption - so why not fit one

 

[Andrew Prince]

It's about robustness, longevity and power delivery characteristics (and many other factors besides!)

BMW make a 2l turbo-diesel that produces more power and torque (on a test bed) than a 4.2l TD in an 80. But the 4.2 is MUCH less stressed - brake mean effective pressure is lower, compression ratio is lower, operating rpm is generally lower and so on. Your big 4x4 is typically used for towing, slogging around farms, overlanding (more so than a 2l BMW!) which are more demanding on an engine and the inconvenience of engine problems is much more significant off the beaten track than at the side of a tar road. Slow speeds mean cooling is critical, dusty environments mean accelerated engine wear. Offroad use often requires very low engine speeds, good engine braking and plenty of torque available from idle. All characteristics provided by bigger engines generally.

The 4x4 also does more "hours" for the same mileage than a roadcar, which could average 40-60mph over its life. So 200k miles offroad will mean probably double the engine hours compared with 200k miles in your BMW 2l commuter. So you need a big tough engine if you want the truck to last.

Farmers, construction companies and weekend offroad warriors expect their 4x4s to last a decent length of time. So Mr T and his close friends at Nissan stuck in big, unstressed diesel engines in their heavy duty trucks and have earned a great reputation as a result.

I am interested to see how the new VW Amarok pick-up with its high speed (for a diesel) 2l TDi engine stacks up against the Hiluxs etc over time. This is obviously a different concept (lightweight, good fuel economy) to the traditional heavy-duty, lower performance route most truck manufacturers have opted for.

EDIT: As a post script, remember that engines are simply a tool for converting chemical energy in fuel into kinetic energy. In theory if you burn a given quantity of fuel, you generate a certain quantity of energy. Of course in practice we know that there are many inefficiencies, which make some engines produce more or less power than others. The principle still holds - if you can burn as much fuel in a 2l engine as 4l engine, you can produce similar power. The challenge for an engine is whether it can cope with that power for its entire design life in a variety of changing and testing conditions.

Cheers,

 

[Chris]

I guess that squeezing more out of less is done with a healthy dose of cleverness. All sorts of trickery and stuff. This is all good stuff but not the sort of thing that you can fit with a spanner and a lump of wooden fence post. It's all white coats and laptops. Not saying they're more unreliable but they are harder to fix at the roadside. To be uncomplicated and up to the job, I suppose they need to be larger. That makes them less efficient. None of this matter if you don't live in a country where the government robs your eyes out with taxes. Suddenly the US are interested in making cars that have a thing called 'fuel efficiency' as a feature. Never did before cos petrol was just about free.

The Disco only has a 2.8 or something doesn't it? My mate gets 35 mpg out of his. Is it reliable ....

Moving on.

Chris

 

[Nuclear Chicken]

Don't forget too that when the 80 was 'king' the 2l BMW/ VW/ French engines with high power outputs didn't exist.
I guess there is a psychological thing in it too. No-one wants to lift the bonnet of a 80/100/120 whatever and find a tiny capacity engine in there regardless of what the power is. At the end of the day it's hard to beat cc's. I suppose VW's foray into this new pickup thing will be a bit of a tester......Maybe the small engine will one day find it's way in to the Touareg as well....

 

[chriscolleman]

An engine has a sweet spot at which it produces maximum torque and hp.

The powerband on a small bore engine is much narrower than on a large bore engine.

So small bore engine need complicated transmissions, lots of different gears to remain in the sweet spot.

Large bore engines need less complicated trannys for the same result.

Ideal is a small bore engine with minimum mass in the moving parts combined with an infinate variable transmission. Like a 50cc scooter can pull away from you at the lights like nothing else.

Now how would we engineer that to last 1000000 kms in the outback with a minimum of attention??
The Toyota way off course!!

 

[karl webster]

The small turbo engines have more power and top speed lorin.

Sit 3.5t behind them on a trailer all day every day and you would soon understand the reasons

The nissan navara i think are a loverly 4x4 with high performace diesel in them and i have seen the con rod through the side of the engines with around 20,000 on the clock

Thats my view anyway.

Karl

 

[Cptsideways]

I'm sure a 2l Tdi that weighs 300kg less than the 1HD would go quite well in an 80. But there would be no torque at idle to mention. However modern turbo tech can get the thing spooling up 1200rpm

TBH the 1Hz makes very little power for its size, even in the torque factor, however as mentioned its a very low stressed engine much like a truck that will do 500k miles without much bother.

 

[J66P]

simples, no substitute for cubic inches