Why are two stroke engines less efficient than four strokes, in layman's terms?

Basically this, that's why when Juan is leafblowing in front of your house it smells like gas.

would the not having valves change the sound of the engine at all?

Not really, two strokes often sound the way they do because it's usually got one or two cylinders on a bike.

Not necessarily. Its one combustion event per cycle vs one every OTHER cycle. More boom, more power.

Because the exhaust and intake ports are open at the same time, which means some fuel gets wasted. They also pretty much require carburetors (there are some fuel injection attempts, but those are compromises at best) and they burn oil by design.

I'll add onto the others; they also typically operate best at higher RPM's than four-strokes

It is relatively outdated outside of special purposes like snowmobiles, where lower weight, high powerband & simplicity are essential

That's actually not true, when comparing engines of the same type (for example motocross bikes), 2 strokes can make the same amount of power at lower RPM compared to 4 strokes twice the displacement (notice how the 4 stroke can rev around 1500 RPM higher). They do have a much narrower and less usable powerband though.

1. Air/fuel mixture going out the exhaust
2. Lower expansion ratio because exhaust happens before bdc

I know the point of most petrol 2 strokes is to minimize the number of parts used, but I wonder how efficient and powerful a uniflow, supercharger scavenged, exhaust-valved 2 stroke petrol engine would be?

Basically a 2 stroke diesel engine, but with direct petrol injection and a sparkle plug.

Double the power strokes double the fuel.

They aren't less efficient, unless you're simply comparing power made to fuel consumed without taking into account weight, capacity and cost. What they are is environmentally unfriendly and they rustle greenies hard.

fun fact
4 stroke is actually older than 2 stroke

>literally sends unburned fuel and oil out the exhaust
>it’s not less efficient
Okay

>tfw no two-stroke neon

Why couldn't 2 stroke engines utilise a single exhaust valve per cylinder to prevent fuel loss?

>YZ250 makes 47hp from 250cc
>YZ250f makes 39hp from 250cc
>YZ250 is ~5lb lighter

>confusing efficiency with power output

Which measure of efficiency are you using user? Because they are more efficient at making power with a given weight and more efficient at making power with a given capacity, both of which are critical in racing applications, whereas fuel efficiency is not, in the least.

They do on diesel 2 strokes, but 99% of 2 stroke engines are made with the intention of being cheap, i.e. having minimal parts.

If you wanted valves, you'd need a shitload more parts and a separate pressurized oil system to lubricate the camshaft and associated components, since the fuel-oil mixture used to lubricate the crankcase components wouldn't go anywhere near the cam.

>2 stroke valveless engine:
>crank
>piston
>crankcase
>cylinder head
>reed valves

>2 stroke engine with exhaust valves - all of the above components, plus:
>camshaft
>camshaft bearings and bearing caps
>timing chain/belt
>sprockets for chain/belt
>valves
>valve springs and collets
>valve guides and seals
>rocker cover and gasket
>lifters, maybe pushrods if OHV
>oil pump
>oil reservoir
>oil pressure regulator
>oil filter

That's a hell of a lot more parts to manufacture. If you wanted to lubricate the crank and conrods with your oil pump (eliminating the need to premix your fuel and reducing emissions), you'll also need a supercharger to scavenge instead of using the crankcase, which is even more parts.

Why does the green go in the spin part?

You'd be better off with a 4 stroke at that point

blowing fuel out the exhaust was a side effect of making good power, but with modern direct injection they are getting much better in that regard.

From what I understand, modern two stokes do use direct injection and lubricate with oil separate to the fuel.

Two stroke engines are not less efficient than four stroke engines. It is the design most cheap 2-strokes have that makes them less efficient. The design OP posted is such a design. A good amount of fuel is blown out of the exhaust, and lubrication is a compromise between a seized piston and smelling like India.
However, a 2-stroke engin does not have to be designed like that. Most freighter ship engines are actually 2-stroke diesels.
de.wikipedia.org/wiki/Zweitakt-Dieselmotor#/media/File:Diesel_engine_Uniflow.PNG
The inlet ports are at the bottom of the cylinder. They are fed with pro-pressurized air. At the top there is a single exhaust valve.
Achates power (animation related) developed a 2-stroke opposed piston diesel for the F-150.
youtube.com/watch?v=ZyrBoGzhaVE

air cooled, friend

Efficiency is making the most power with the least waste. Just because you're making more power doesn't mean you're creating more waste too. Racing engines are inefficient for a reason, because power is more important.

Because the air/fuel/oil mixture also lubricates the crank bearings, and the scavenging pulls the mix into the combustion chamber.

Are you silly?

the most efficient engine in the world is a diesel 2 stroke.

The 2 problems with most gasoline 2 strokes is that they mix the fuel with exhaust, wasting it, and that they get their oiling through the air fuel system, and that motor oil is a shitty fuel despite chemical engineer's best efforts.

I should've specified that I meant two-strokes often like to work in their higher rev ranges, whereas four-strokes often have a more broad powerband. This is very noticeable when you compare engines in the same size (125cc two-stroke, 125cc four-stroke) in the same type of vehicle).

The fun thing is that you can actually heavily change the powerband of the two-stroke with exhaust modifications. I've drive the same moped with two different exhausts where the "power kick" would come in and also end about 2500rpm sooner than with the stock exhaust

I miss my old two-stroke mopeds...

well with two strokes changing the exhaust resonant frequency is just like changing the valve timing, so in a way that's like going VTEK.

they sound different because they have big expansion chambers which basically echo the exhaust back into the cylinders. They also fire twice as often per cylinder per rpm.

Listened to a talk by the global Powertrain director of Chrysler during those times. The Neons were for testing, not necessarily for that car.

No, efficiency entirely depends on the metric you're measuring. The particular metric you are using is fuel efficiency.
A truck engine is efficient at moving weight in comparison to a light duty petrol, it's inefficient at moving itself.

You say that but the average small capacity 4 stroke revs to 14k and produces peak power somewhere at 13k. 2 Strokes don't need to rev anywhere near that to produce more power and as you mentioned you can tune where you want your power in a 2 stroke with ease, much more difficult with a 4 stroke.

> the oil gets wasted
you mean consumed. it's only wasted if that's not part of the design

Waste most of your rotating inertia by getting rid of a dedicated exhaust stroke

Two stokes turn the crank one rotation for every two combustion cycles. Have any four stroke RPM and multiply by 2 and you got a two stroke engine.

Except a rocket does not have an engine, it has a motor.

lower compression and incomplete combustion

>an entire machine that burns gas to run a fan because you don't feel like using a rake
what asshole invented this?

Make a pneumatic exhaust that alters back pressure

That exists. Variable exhaust ports on two strokes were quite common