Thread: a question from my auto exam

Originally Posted by Slunnie

In racing circles, the 4 strokes are now able to produce more power than a 2 stroke for a given capacity. The problem with a 2-stroke is that even with expansion chambers they are still not an efficient motor. The other limitation is that 2-strokes is that they cant really make peak power happen beyond about 11,000 rpm, though they can make it linger well beyond that rather than falling off. 4-strokes they can make pull for as long as things remain in synch. I think F1 are hitting around 18,000rpm and I'm not sure what the bikes are doing these days.

The biggest limiting factor in a modern 2stroke is---

They no longer race them,,,,

yes.
that old chestnut " racing improves the breed" IS true.

Originally Posted by Blknight.aus

Starting at TDC (assuming a single cylinder over head valve configuration)

With both valves on the rock, inlet opening exhaust closing the next downward movement of the cylinder is called the induction stroke which is achieved initially via an external force applied to the crankshaft to initiat rotation and once the OTTO cycle is established under inertia gained during the power stroke. The inlet valve opens rapidly and at the bottom of this stroke the inlet valve closes and piston begins to move up the cylinder. At this point a fuel/air charge (in the case of a petrol) has been administered to the cylinder and is ready for compression (in the case of a diesel its only air)

The next stroke is the compression stroke there is no valve movement on this stroke of the cylinder and the piston is forced up via the rotation of the crankshaft via an externally applied force or under the inertia of itself, the flywheel and any other rotating mass attached. Near the top of this stroke the spark plug is fed a high voltage from the ignition system which arcs across the airgap and commences combustion The piston is still moving upwards when this occours. In the case of a Diesel the compression is sufficient to raise the air temperature above the flash point of the fuel that is injected in atomised form into the combustion chamber. This injection occours at about the same timing as the spark in a petrol engine.

Once the crank rotates past TDC with both valves closed the power stroke (assuming the combustion process has occurred and is self sustaining) the power stroke has begun which takes advantage of the adiabatic process where a rise in temperature gives a rise in pressure, The rise in pressure exerts force on the crown of the piston and in following with the isobaric rules for thermal reactions pushes the piston down (AKA nature abhors a vacuum) As the piston approaches bottom dead center the exhuast valve begins to open and the residual energy of the combustion process begins the process of evacuating the combustion area.

Once the cylinder has passed bottom dead center and is rising again the exhaust valve rapidly opens and the piston again under inertia or external force applied to the crank shaft rises and expels the remaining gases. Near the top of this stroke the exhaust valve begins to close and the inlet valve begins to open in preperation for the cycle to begin again. This forth and final stroke is called the exhaust stroke.


Is that close enough or do you want an indepth answer?

Dave, have a cup of tea, a Bex, and a nice lie down. Failing that, take a laxative.

And if you want some really interesting engines, I seem to remember from the 1920s a marine engine that ran steam below the pistons, and diesel above, with steam being raised by the diesel exhaust.

John

Originally Posted by Blknight.aus

not always...

Ive seen 2 strokes with timing chains and over head valves.

Yes, you are quite right, I was talking in general in what most of us will see in petrol and generally in motorcycle / buggy type engines. I was not including rotary valves only OH valves. Who could forget the 2 stroke Detroit Diesels, not that many of us will actually see to many close up let alone work on them, but I do love them.
Actually only rebuilt the kids 2 stroke quad this week. What a pleasure and simple task they are to work on. Complete big bore kit installed in 45 minutes. The 4 strokes I am used to would have taken all day or more.
As per another post a Rotary is not a 2 stroke in the true sense as it works in a completely different way and application. My understanding is the Rotary Wankel works a bit more like a 4 stroke than a 2 stroke with Inlet, Compression, Combustion & Exhaust with each task occuring independentlly, but I also dont know too much about these, so I could be way off base. Where as a 2 stroke has two different task occuring at each stroke.

Originally Posted by Pedro_The_Swift

The biggest limiting factor in a modern 2stroke is---

They no longer race them,,,,

yes.
that old chestnut " racing improves the breed" IS true.

True in the case of Motor-X yes which is very sad but the old 250 2 smokers can not keep up with 450 thumpers (I own a 450 and have ridden many 250 2s).

But in the boating world Evenrude turned the game on its head the E-tec.
Fuel injected 2 stoke outboard.
The two-stroke lives! : Evinrude's E - TEC engines have done the seemingly impossible; saving the air-cooled two-stroke from oblivion | Automotive Design & Production | Find Articles at BNET

Ben

Originally Posted by CraigE

Actually only rebuilt the kids 2 stroke quad this week. What a pleasure and simple task they are to work on. Complete big bore kit installed in 45 minutes. The 4 strokes I am used to would have taken all day or more.

I can't accept that unfair criticism of 4 strokes.

When I used to run my SL125 Honda (bored and stroked to 175cc) on petrol during the week and alcohol when racing on Sundays, it used to take me less than an hour to do the swap.

That included:

• removing the fuel tank
• removing the head and barrel
• removing the piston
• fitting the higher (or lower) compression piston.
• replacing the barrel, head and fuel tank
• remove the bottom of the carby
• replace the main jet with on that was 40% bigger (or smaller)
• lift the carby slide out and move the circlip down (or up) three notches
• drain the petrol (or alcohol) and replace with alcohol (or petrol)

I used to think that was reasonably slick but a friend of mine who was a mechanic and had access to a better range of tools than I did could do that same swap at the start of his lunch break and still have time to ride home across town for a bite to eat and get back in time for the afternoon.

I reckon with the right tools, I could have given you a run for your money with your 45 minute 2 stroke changeover.

Originally Posted by schmierer LR at singleton

good answers another question was name 2 advantages that a 2-stroke engine has over a 4-stroke engine once u guys tell me your answers ill tell you mine just in case im wrong

1. They start easier than a 4 stroke after sitting, neglected, in the shed for a year.
2. They smell like trailbikes.

Originally Posted by Sleepy

1. They start easier than a 4 stroke after sitting, neglected, in the shed for a year.
2. They smell like trailbikes.

we have an old 1980 kv 250 4-stroke that we leave for olng periods of time and it generally starts within the first 5 kicks