Better mileage 2-stroke petrol or diesel

its is known that four stroke diesel has better mileage than four-stroke petrol but if we compair 4-stroke and 2-stroke diesel then which has good mileage and similarly 4-stroke and 2-stroke petrol engine which one has better mileage in all four engine

Bump. Don’t have clue.

I would guess the two stroke based on the fact that nearly all really large diesel engines, ships, locomotive, etc, are two stroke. Fewer cylinders needed for a given power output means less friction.
Unlike spark ignition engines, overscavenging does not result in unburned fuel going out the exhaust in a two stroke diesel engine.
In small engines, perhaps four stroke engines may have an edge due to the fact that a turbocharger turns the intake stroke into a second power stroke. Most all small diesel engines, trucks, cars, etc, are four stroke.

It’s really academic since 2 stroke engines cannot meet stringent exhaust emissions we have today. That’s why the 2 stroke lawn mowers are history.

4 stroke gas and diesel get better mileage because 4 strokes have greater efficiency than 2 strokes. Think of it this way, 2 strokes of the same engine size make more power than 4 strokes, but not twice the power.

4 stroke engines don’t need to share functions during each stroke to operate. Suck, squeeze, bang, blow, the 4 stroke cycle need to be combined into suck-squeeze and bang-blow for 2 cycles compromising power (the 2x power is more like 1.4x) and efficiency

Like B.L.E. says.
IIRC 2 stroke diesels were sometimes used in large trucks.
This big guy is 2 stroke turbocharged.

Two stroke gas engines were known for better power/pound of engine weight, but the ones I remember were not better for mpgs. No modern ones to use for comparison.

But I’m pretty sure the most efficient engines ever built were 2-stroke diesel Prime Movers. It seems they do better in static output cases, where power and RPM a fixed quantites. I suspect you can model the fluid motion fairly well for one condition and design for it, but a range of RPMs and power is too much.

Yep, they’re used for ships:

"Designed to provide the motive force for a variety of supertankers and container ships, it comes in 6 cylinder in-line through to a whopping 14 cylinder version. The cylinder bore is 38 inches and the stroke is just over 98 inches. Each cylinder displaces 111,143 cubic inches (1820 litres) and produces 7780 horsepower. Total displacement comes out to 1,556,002 cubic inches (25,480 litres) for the 14-cylinder version.

At a length of 89 feet and a height of 44 feet, the total engine weight is 2300 tons - the crankshaft alone weighs 300 tons.

The RTA96C-14 can achieve a maximum power output of 108,920 hp at 102 rpm and astonishingly, at maximum economy the engine exceeds 50% thermal efficiency. That means, more than 50% of the energy in the fuel is converted to motion. Its Brake Specific Fuel Consumption (BSFC) at maximum power is 0.278 lbs/hp/hr. Comparatively, most automotive and small aircraft engines can only achieve BSFC figures in the 0.40-0.60 lbs/hp/hr range and 25-30% thermal efficiency range."

I ran accross this.

it looked interesting. Direct injection gasoline 2 stroke is vastly different than carberated chain saw technology.

The vast majority of heavy trucks are 4 stroke diesels rather than 2 strokes because of fuel economy and a broad horsepower and torque band.
The 2 strokes only advantage is lighter weight.
However if you are running a diesel-electric like a locomotive where the diesel runs at a constant rpm, you don’t have to worry about a broad powerband or how much torque drops with rpm. Seems like a two-stroke diesel-electric hybrid would work.

Direct injected turbocharged 2-strokes, gas or diesel, can meet emissions and post some astonishing efficiency numbers…It will take $10/gallon fuel before these designs are marketed, probably in hybrid form, achieving 100 MPG …

Diesel engine have always used direct injection, thus in diesel engines, the two stroke design doesn’t have any particular pollution disadvantages that four stroke engines don’t also have. If a diesel two stroke over-scavenges the cylinder, all that is lost is perfectly good fresh air, not unburned fuel.
Over scavenging is the reason gasoline two stroke engines wasted fuel and were dirty.

Also, two stroke diesels normally use a separate blower for the scavenging air instead of using the crankcase as an air pump. This means that they have oil in the sump just like a four stroke, no mixing oil with fuel.

Over scavenging in a diesel doesn’t just throw out fresh air. Some energy is lost in moving the air. And over scavenging is necessary to ensure that there’s enough air to oxidize all the fuel. I think that’s the reason why newer trucks run on for stroke inline six rather than two stroke V8s.

A marine engine is optimized to operate in a very narrow range of speed (0-100 rpm). It also has to produce a lot of power in the lightest possible package because it is a part of a floating system. A four stroke would probably be too heavy. BTW, just one of those round things that hangs on the wing of a Boeing 777-300er produces more kilowatt than the biggest marine engine. While it’s not as efficient, it fits the system it’s in.

Marine engines in freighters and tankers are specialized around the fuel (cheap heavy oil) and thermal efficiency. Because of their size (like 40,000 hp from 6 cylinders) they have to be slow turning. The actual weight of a marine diesel is of little consequence compared to the total weight of the ship. Efficiency and the ability to use cheap fuel is most important.

Cruise ships use multiple but smaller engines and often have electric drives so they can cruise at any speed from a slow crawl for whale watching to full speed when crossing the ocean…

The auxiliary power on ships is normally supplied by multiple smaller diesels.

In military vessels, light weight is more important than operating costs, and cruisers often use gas turbines.

Love the 6.71 diesel in city buses. The exhaust pressure could knock you over. It also had a good type of supercharger for a 426 Hemi.

Most industrial two stroke diesels use a form of inline scavenging instead of the loop scavenging used in gasoline two stroke engines.
The 6-71, for example, has poppet valves in the head just like a four stroke engine, but they are all exhaust valves, the intake ports are a ring of holes near the bottom of the cylinder. As the piston approaches BDC, the valves in the head open to release the exhaust and the intake ports are uncovered by the piston and the fresh air enters from the bottom.
With cam opened exhaust valves, the valves can be timed to open before the intake ports, this is called the blowdown part of the cycle. Then they can actually close before the intake ports allowing a slight pressurization of the cylinder.

Even with four stroke engines, the intake and exhaust periods are not as separate as many people think. There is a period called the overlap where the intake and exhaust valves are open simultaneously. This can allow over scavenging, especially when a turbocharger is pressurizing the intake manifold.
On gasoline engines, the overlap period can allow the vacuum in the intake manifold to suck exhaust through the cylinder and into the intake manifold during part throttle operation.
The characteristic rough surging idle of a race car engine is mostly due to the extreme amount of valve timing overlap and this has the intake charge so diluted with exhaust that it idles like a two stroke.

As a point of interest, here’s the valve timing of a Kawasaki Ninja 300 two cylinder motorcycle engine.
Intake open 36 degrees BTDC
Intake close 56 degrees ABDC
Duration 272 degrees

Exhaust open 61 degrees BBDC
Exhaust close 31 degrees ATDC
Duration 272 degrees

There is 67 degrees of engine rotation where the intake and exhaust valves are open simultaneously.
Yet this engine ticks over like a mildly tuned car engine. How so? Each cylinder has its own throttle that is very close to the intake port and so the vacuum at idle exists as pulses instead of the steady vacuum of a multi cylinder engine using a single throttle. By the time the intake valve opens, the vacuum from the previous intake stroke has largely died away and so there is very little suck-back of exhaust through the simultaneously open valves.
I remember someone who built a custom manifold for a Yamaha 650 twin so he could feed both cylinders with a single carb instead of having a separate carb for each cylinder. This change by itself resulted in the stereotypical dragster idle due to the steady intake manifold vacuum sucking exhaust back through the engine during the overlap period.

Wow BLE,that is good info,so thats what overlap is,does that have anything to do with reversion?