Pulled or pushed ?

[Chris Channon]

Hi all, just a quickie. The fuel in a four stroke model engine, is it pulled in by the down going piston or pushed in by atmospheric pressure ?
Thank you
Chris C

[Geoff S]

I would say both because suction is really just removing air and atmospheric pressure (in the absence of a pump) tries to refill the void.

Geoff

[Jon H]

technically I think its pushed.

If you look at it like being in space with no pressure outside the spaceship and high pressure in it, if you open the door you will be blown into space and not sucked into space.

[Dave Bran]

I have it on good authority the fuel doesn't care.....................

[trebor]

This could turn into another poll.

[Dave Hopkin]

Don't see how it could be "sucked in by the down going piston" - that would exert suction on the carb output and air would be far easier to suck in that fuel.................

There might well be an element of suction when running from the needle valve as air is dragged past the spray bar

Seems logical that air displaced by the exhaust stroke would provide the bulk of the pressure to drive the fuel

there might also be an element of siphoning too............

[Peter Miller]

The old, and I mean OLD description of the operating sequence for a four stroke engine is:

SUCK, SQUEEZE. BANG .BLOW

[Martin Harris - Moderator]

Surely the fuel is pushed in by atmospheric pressure - I'm sure I recall my physics teacher telling us that suction didn't actually exist as a force...

[iqon]

Whatever you decide, I`ll go with the flow

[PatMc]

Posted by Martin Harris on 29/12/2017 09:19:45:

Surely the fuel is pushed in by atmospheric pressure - I'm sure I recall my physics teacher telling us that suction didn't actually exist as a force...

My physics teacher read from the same hymn book.

Google "Magdeburg Hemispheres" for the definitive explanation.

[Plummet]

As the piston goes down (assuming the engine is not running inverted ) it tries to create a vacuum in the engine. The pressure in the engine becomes lower. But the input valve is open, so air at atmospheric pressure, which is higher than the internal pressure, is blown into the engine, through the carburetor by the atmospheric pressure. Thus the air in the carburettor is at a pressure less than atmospheric. The fuel, assuming it is level with the carb, will be at atmospheric pressure, so there is a difference of pressure between the fuel and the carburetor, so that the fuel is blown into the carb by atmospheric pressure.

To add to that effect, air that is moving is at a lower pressure than the static air around it (as demonstrated in the first of the 2017 RI Christmas Lectures by the use of big balloons and a leaf blower), so that the air pressure in the moving air in the carb is at a still lower pressure, increasing the pressure difference with the fuel in the tank, and increasing fuel flow.

This reminds me of an excuse for over imbibing. If you drink your booze through a straw then you can claim that it is not your fault - it is not you sucking in the drink, it is the atmospheric pressure forcing it in to you,

Plummet

[DH 82A]

The old saying " Nature abhors a vacuum ". ( showing my age there ).

[Don Fry]

I am glad some of you listened to your physics teachers.

[Biggles' Elder Brother - Moderator]

I think this is one of those it depends how you want to view it things. It's really a question of "what is the actual positive acting force" as opposed to "what is the reaction to that force" or "what is the causal driver behind the action of that force" - OK, clear as mud I know! So let's try it different way,...

My physics teacher was fond of reminding us that there is no such thing as "cold" - there is only the absence of heat. When you open the door you do not "let the cold in" you actually let the heat out. Heat is a form of energy - a real tangible thing. Cold is simply what you have when you don't have enough heat.

The situation is similar with vacuum. A vacuum is simply a area of lower pressure than its surroundings - it's the relative absence of air molecules. You could then say there is no such thing as a vacuum, just an absence of pressure. A vacuum is therefore a "negative thing" - like cold it is the absence of something, not a tangible thing in its own right. Just as heat will seek to warm a cold area, so positive pressure will seek to fill a vacuum - nature abhors a vacuum! So I suppose you could say that the low pressure, caused by the descending piston, is our vacuum and the atmosphere outside is our source of positive restoring action that will try to equalise that vacuum. The vacuum is the cold, the atmosphere is the heat. In which case you'd have to say "it's pushed"!

BEB

Edited By Biggles' Elder Brother - Moderator on 29/12/2017 13:05:14

[Geoff S]

Posted by Peter Miller on 29/12/2017 09:16:49:

The old, and I mean OLD description of the operating sequence for a four stroke engine is:

SUCK, SQUEEZE. BANG .BLOW

Actually that sequence describes the operation of all combustion engines.

I had a mouse pad from RR which used those terms to describe the operation of a gas turbine - it's just that everything happens all the time rather than sequentially.

Unfortunately, the picture gradually disintegrated and all that remains is the base ... and this picture

Geoff

[cymaz]

Posted by Chris Channon on 28/12/2017 23:04:05:
Hi all, just a quickie. The fuel in a four stroke model engine, is it pulled in by the down going piston or pushed in by atmospheric pressure ?
Thank you
Chris C

As I visualise it fuel is helped out on the tank by a bit of air pressure, ( either by exhaust or in the case of Laser prop wash) and low pressure created by the piston going down the cylinder. The Venturi in the carb helps this along. I would say most of the fuel flow is the draw of the downward motion of the piston.

Am I correct, misinformed or just plain stupid

[Paul C.]

There is also a period of valve overlap at the top of the exhaust stroke. The inlet valve opens just before the exhaust Valve has closed, the effect is to help draw in the fuel air mix on the induction stroke. This is even more effective if a turbo or supercharger is fitted, you have just got to love 4 strokes 👍

Paul. �

[Simon Chaddock]

Now I wonder what effect the carburettor venturi has on the fuel flow?.

[Kevan Gillings 2]

HI, the Venturi in the carb produces Bernoullis therum, ie air flowing through a restriction speeds up, therefor causing a drop in air pressure, the fuel at the jet tries to fill the low pressure due to atmospheric pressure acting on it

, so we get a mixture of fuel and air.

[Former Member]

[This posting has been removed]

[Stephen Smith 14]

Posted by Tom Sharp 2 on 29/12/2017 18:49:31:

Posted by DH 82A on 29/12/2017 11:44:46:

The old saying " Nature abhors a vacuum ". ( showing my age there ).

My dear old dad was always quoting that saying.

still don't know what abhors means

Regard with disgust and hatred