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4WD vs. AWD. What's the Difference?

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Re: 4WD vs. AWD. What's the Difference?

#181

Post by sliverstorm »

lb1lf wrote: post #109
wil421 wrote: post #105

Earlier quoted context omitted.

The parent and other people commenting about trashing 4WD systems are from Europe. I'm not sure if there are as many remote places in Europe anymore especially compared to Utah. I go trout fishing a lot in North Georgia and many places I go would be accessible in a small fwd car. In fact if people saw you driving an incapable vehicle they would stop and warm you about proceeding further.

Try Norway for size. I live in a rural area and the everyday car is a Land Cruiser - in summer, just about anything goes - but come winter, and big, studded tyres, low range and diff locks all over (well, center and rear in my case) are what gets you from A to B and back. Anecdotal evidence makes me claim that electronic traction control systems are excellent, but still no match for good ol' manual systems - probably…

Electronic systems also work best on the road. The looser conditions get, the worse they often perform.

Re: 4WD vs. AWD. What's the Difference?

#182

Post by avn2109 »

sunstone wrote: post #33
japhyr wrote: post #3

Here are the key takeaways for me: "Thanks to that differential between your axles, an AWD car will send your engine’s power down the path of least resistance—the wheel with the least grip. Where a two-wheel drive car can only choose between two wheels, an AWD system looks for that least resistance across all four wheels." "4WD works by locking the front and rear axles together, splitting torque 50:50 between them. T…

And in the next paragraph "To counteract this [sending of power to the wheel of least resistance] , the better AWD cars are fitted with a center differential that contains a clutch or viscous drive unit. This splits torque front-to-rear, directing it away from the spinning wheel. " So, a well engineered AWD system actually sends power away from the slipping wheel(s) and towards the wheels with traction, which makes m…

>> "...sends power away from the slipping wheel(s)..."

Exactly this. The best ones are entirely mechanical, without any fancy computers or even viscous couplings. Audi sold (sells?) IMHO the most sophisticated AWD system on the market, which uses a system of crossed helical gears to continuously and instantaneously redistribute torque proportionally to wheel traction, up to a roughly 80-20 split. There are no computers and the whole thing is mechanical. The differential is a minor mechanical miracle sold under the trade name Torsen, short for "torque sensing." Despite Audi's German-engineering themed marketing, the differential was actually designed by an American.

In the 80's, the station wagons even had a manually-actuated center locker, which was eventually replaced by a servomoter driven locker. I don't know what the new ones have.

Re: 4WD vs. AWD. What's the Difference?

#184

Post by unusximmortalis »

kls wrote: post #161
tiglionabbit wrote: post #103

Earlier quoted context omitted.

> That's right. A poorly-engineered 4WD system means you are stuck on a hill because one wheel is on slippery wet leaves. AWD should mean you can get up that hill with three wheels on slippery wet leaves, and one with grip. That's the simplest explanation. I think you just said the opposite of the guys above you while saying you agree. 4WD should not have a problem on slippery terrain.

It actually has nothing to do with 4WD or AWD, the article is poorly written and confuses several terms. At the surface 4WD is manually selected and AWD is 4WD all the time. Thats it they are the same thing, it's just 4WD means you opt to put the vehicle in 4WD. The term the are talking about when traction is applied to the wheel with the most binding to the ground and away from the one that has the least traction is…

I found the article very good explaining the basics and history of awd vs 4wd. What you explain is the status of today's 4wd vs awd, where indeed the features in awd vs 4wd cars can overlap a lot and because of the big variety of implementations quite hard to understand by regular Joe

Re: 4WD vs. AWD. What's the Difference?

#186

Post by mod »

kilovoltaire wrote: post #58

Wow, early cars connected the engine to only one wheel, to completely avoid issues of differential. (According to the cool old video linked in the article.)

My cousin had a firebird, I think a '67, that (seemingly?) had 1wd. He could do donuts that left one trail.

Not sure if by design or some kind of failure...

Re: 4WD vs. AWD. What's the Difference?

#187

Post by fnj »

esaym wrote: post #164
magicbuzz wrote: post #129

Earlier quoted context omitted.

Jet engines are not "efficient". The SFC is worse than piston engines and in fact, the smaller the turbine the worse the efficiency: https://en.wikipedia.org/wiki/Brake_specific_fuel_consumptio...

I've never heard that jet engines are less efficient. From what I remember piston engines are 33% thermally efficient, diesels 36% and turbine engines are upwards of 60%+. Of course there are many types of jet engines and ways to get power off of them.

You heard wrong. Diesels under optimum conditions can easily exceed 45%, and state of the art is right around 50%. Gasoline engines can reach 40% or better (again, under optimum conditions). Turboshafts are generally around 35-40% efficiency at best-economy conditions, and dismal under low-load conditions. The very largest turboshafts can reach 40-45%, but come nowhere near to diesel efficiency at low load.

In all cases, you can increase efficiency somewhat by going to the complication of compound cycle, where you harness some of the wasted heat in the exhaust by making steam.

Large, high bypass turbofan jet engines are very efficient from an overall propulsive efficiency standpoint, but you aren't measuring the same thing (shaft power times time, divided by energy consumption).

Re: 4WD vs. AWD. What's the Difference?

#188

Post by fnj »

mikeash wrote: post #116
dredmorbius wrote: post #112

Earlier quoted context omitted.

Gas turbines operate efficiently at constant output, which a hybrid-drive system with storage can manage. They only need to kick out enough power, on average, to keep the storage system fully charged. Batteries and eletric motors can handle both high-demand accelleration, and regenerative braking. Since the turbines are running at a very constant range, and can be rated to average out power load inclusive of idle tim…

For anyone interested in turbines and cars, there's a delightful episode of Jay Leno's Garage about a Chrysler turbine car: https://www.youtube.com/watch?v=b2A5ijU3Ivs He talks extensively about how it works, what it's like to drive, and the various difficulties that resulted in it never entering mass production.

It sounded like a gigantic loud vacuum cleaner even just idling, and gave the impression of straining mightily to achieve even mild acceleration. Fuel consumption was TERRIBLE.

Re: 4WD vs. AWD. What's the Difference?

#189

Post by fnj »

samwillis wrote: post #41
mschuster91 wrote: post #26

I wonder why car manufacturers still use direct force transmission with hugely complex (and in case of ignoring the instructions, outright dangerous) like 4WD/AWD instead of diesel-electric or diesel-hydraulic systems like on locomotives. A computer could instantly shift torque with both of the hybrid solutions, you wouldn't need a clutch or a gearbox any more, not to mention cheap all-wheel-drive including the trail…

The Jaguar C-X75 [1] concept car uses exactly this idea, it uses a tiny and very efficient jet engine that is charging the batteries. Very neat idea. 1: https://en.wikipedia.org/wiki/Jaguar_C-X75

1) That's not a "jet engine". It's a turboshaft. A turboshaft is a gas turbine producing its power mechanically at the shaft. A turbojet is a gas turbine producing its power in the form of reaction thrust out the tailpipe.

2) Gas turbines scaled down to small size (let alone "micro" size) generally have dismal thermal efficiency. All the most efficient gas turbines are gigantic (tens of thousands of hp).

Re: 4WD vs. AWD. What's the Difference?

#190

Post by fnj »

dredmorbius wrote: post #112
mschuster91 wrote: post #70

Earlier quoted context omitted.

What the f..k. Thanks for that link, a jet engine?! That's crazy. But it's the "awesome" kind of crazy. And the CO2 values... wow, how is that possible? Burning a defined amount of fuel should always produce the same amount of CO2?

Gas turbines operate efficiently at constant output, which a hybrid-drive system with storage can manage. They only need to kick out enough power, on average, to keep the storage system fully charged. Batteries and eletric motors can handle both high-demand accelleration, and regenerative braking. Since the turbines are running at a very constant range, and can be rated to average out power load inclusive of idle tim…

The large volume of very hot exhaust you mention is your clue that the thermal efficiency of small gas turbines is poor to dismal.
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