Automotive Automatic Gearbox

[frank brownParticipant @frankbrown22225]

Thanks Ray, you have explained a mystery to me. When I moved house, I did about 20 trips to a storage site about 20 miles away mainly by dual carriageway. I kept the same moderate speed 50 MPH+-1 for all the journeys. So my estate went out every morning laden with materials from my workshop and did 50.6 MPG. On the way back (unladen) it did 50.2 MPG. These trips were in April so the weather was cool and misty going out and fairly sunny coming back. It looks like even a moderate amount of dampness does make a measurable difference in MPG,

Frank

[Ian S CParticipant @iansc]

Perhaps your home trip has more up hill, here in Canterbury (NZ) is a large area of alluvial plains, it's flat to the casual view, but I live 40K to the west of Christchurch, and we are about 650ft higher than Christchurch city, and it make quite a difference to fuel consumption.

Ian S C

[MikeParticipant @mike89748]

This often happens on journeys here in the North of Scotland. On one of my regular 10-mile journeys to a nearby town I get 32mpg going, and nearly 50 coming back.

[John ReeseParticipant @johnreese12848]

The gasoline engine is more efficient when the throttle plate fully open. The modern engine and transmission controllers try to approach that. When I was at the university in the late '50's we dyno tested a Ford 6, probably 240 cubic inches. Its best specific fuel consumption (pound per brake horsepower hour) was at 800 rpm with the throttle fully open. Unfortunately, most of the carburetors made in the US at that time enriched the fuel mixture when it sensed low manifold vacuum due to open throttle at low rpms. That was done to prevent knock on acceleration. There was not the engine/transmission management technology at that time to allow low horsepower operation with an open (mostly) throttle.

One of the biggest energy losses in a gasoline engine is drawing air past a partially closed throttle plate. One of the reasons a diesel engine is so efficient is that it operates without a throttle plate.

[Neil WyattModerator @neilwyatt]

Posted by Paul Lousick on 01/04/2016 22:21:24:

Neil.

Back in the early 80's we used to be able to buy a water injection unit that added a small amount of water to the air/petrol mixture from the carburetor. Supposedly to cause a better fuel burn. I also remember something about a similar system being used in early fighter plane engines.

Paul.

The Focke Wulfe 190 used water injection to achieve 'emergency power' for short periods of time, I'm sure other planes did as well.

[Neil WyattModerator @neilwyatt]

Posted by Mike on 02/04/2016 10:06:59:

my petrol Renault Megane has the same gearstick and arrow picture to tell me when to change gear, and it advises changing up at surprisingly low revs – usually well under 2,500.

At constant speed to maximise fuel efficiency the cut for my Mondeo seems to be 2000 rpm

Why a six speed box? Hills, I guess

Neil

Edited By Neil Wyatt on 04/04/2016 09:02:54

[herbert punterParticipant @herbertpunter99795]

Posted by Neil Wyatt on 04/04/2016 08:58:11:

Posted by Paul Lousick on 01/04/2016 22:21:24:

Neil.

Back in the early 80's we used to be able to buy a water injection unit that added a small amount of water to the air/petrol mixture from the carburetor. Supposedly to cause a better fuel burn. I also remember something about a similar system being used in early fighter plane engines.

Paul.

The Focke Wulfe 190 used water injection to achieve 'emergency power' for short periods of time, I'm sure other planes did as well.

The Harrier used a lot of water injection to boost the power as you will be able to see when you go to Brooklands

[Howard LewisParticipant @howardlewis46836]

In the old days, trawler engines used a drip feed of water into the induction, when operating at high loads.

Possibly, humidity in the air,(moisture) evaporates in the induction system and by cooling increases air density, and so the mass of oxygen available for combustion. A sort of cheap internal charge cooling. But overdone, can result in water finding its way into the sump and emulsifying the oil.

The Full Load torque curve of a diesel engine is usually mirrored by the Specific Fuel Consumption curve, so the best speed for economy is often Peak Torque speed. Possibly this is the engine speed for maximum volumetric efficiency. Road load operation will mean that the engine is unlikely to be on full load, unless on a hill with a convenient gradient, but can still be running at the speed for optimum volumetric efficiency.

Mechanically governed Diesel engines, having higher compression ratio and miminmal induction restriction were more economical than a petrol engine, plus the fact that fuel was cut off as soon as the engine speed exceeded that selected by the position of the throttle lever.

A Diesel engine with a vacuum governor is a slightly different kettle of fish. It still cuts off fuel on the overrun, and engine brakes because of the induction restriction, but gains efficiency from having less variation in temperature at the Inlet Valve.

Possibly, the best point at which to change up is just about peak torque speed.

Certainly, on a hill hanging on below peak torque is likely to result in a rapid loss of engine speed.

Modern petrol engines, with electronic control, now do the same thing, giving improved fuel consumption. (A closed throttle on a carburetted engine applies maximum vacuum to the idling system and pulls fuel through giving a richer than normal mixture. Despite being a lot heavier, my 2005 1300 petrol gives a fuel consumption nearly 50% better than a 1977 1650 with a carburettor, to prove that point. Having a higher Compression Ratio, and D O H C rather than push rod valve operation helps, despite the pumping losses imposed by the emission control bits.

Howard

[davebParticipant @daveb17630]

Anyone who rode a motorbike in the 60s (and probably well before that) would have noticed smoother running and more power in cool/misty conditions

Dave.

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