By “two stroke” I am referring to the 2-stroke, aka 2-cycle, engine design rather than the number of cylinders an engine has. Nowadays, it seems that every car has a 4-stroke engine. 2 strokes are, or were, common in the marine world as well as small utility engines such as in lawnmowers and chainsaws.
In the old days, some carmakers made cars with 2-stroke engines. Saab was one of the companies that made them, and one model was even sold in the US, iirc. One of the advantages was that you get roughly twice the “boom” per revolution of the crankshaft, and thus can build smaller engines (smaller cylinders, or fewer cylinders) and get higher performance than the equivalent 4-stroke engine.
Why hasn’t anyone built a “performance” sports car with a 2 stroke engine? I’ve read that 2-strokes pollute more (possibly meaning that any new 2-stroke car would fail Emissions Testing) and wear out sooner, but is this inherent to the 2-stroke design, or is it more because 2-stroke development has stagnated as the auto industry around the world has spent it’s money on the 4-stroke engine?
they tend to be less efficient overall, for some reasons. one, they don’t breathe very well; a 4 stroke has about 180° of crankshaft rotation to clear the cylinder on the exhaust stroke, and another 180° to pull in a fresh fuel-air charge. A 2-stroke has about 90-100° of crankshaft rotation to do both of those things at the same time. Second, if you have a crankcase-scavenged design, you are going to find it nearly impossible to pass emissions standards even with direct injection thanks to the total-loss oiling. Third, if you go to a blower-scavenged engine like the old Detroit Diesels or the Chrysler EBDI engine, you’re sapping power by running the blower and still have the breathing limitations of a short window to clear and recharge the cylinder.
That link notes that even the closed-crankcase Chrysler engine couldn’t meet the emissions standards of the day.
I’d add that a two stroke engine depends upon oil in the fuel to lubricate the cylinder walls-that causes emission problems and early catalytic converter failure.
Its a good concept (as mentioned , Chrysler tried to develop one in the 1990s).
But, it has been abandoned. The problems could not be overcome.
oh, and there’s another thing… at least in the past, 2 strokes tended to be significantly louder than 4-strokes, owing at least in part to the pressure in the cylinder when the exhaust ports/valves opened. This is a Detroit Diesel 6V-53 (2-stroke diesel, V6, 53 c.i. per cylinder-318 c.i. total)
The design doesn’t make much sense in a car. The biggest advantage of a 2-stroke is the amount of power it develops for its physical size and weight. In a car an extra 100-200 lbs of engine doesn’t make much of a difference.
Disadvantages are many, several of them direct deal breakers for automotive use:
Work well only in a narrow rpm range. The engine is tuned to certain rpm where it is efficient. Outside this narrow rpm band it is dirty and thirsty.
Emissions. In a 2-stroke some of the fuel will always go straight through the engine out the tail pipe.
Lack of torque. Although the 2-stroke develops good max power, it is much weaker at lower rpm. A 2-stroke car has good top speed, but takes a longer time to get there.
Saab, btw, was in big trouble in the early 60’s. The 2-stroke Saab 96 was not selling. They made a deal with Ford and bought 4-stroke V4 engines. They had planned to offer both 2 and 4 stroke engines, but once the 4-stroke came out no one bought 2-stroke Saabs and they were soon discontinued.
Lotus was testing a 2-stroke engine last year or the year before. I haven’t heard anything about it lately though. There is a company in Michigan called EcoMotors that is exploring 2-strokes that are less polluting than ones of old. They haven’t got the attention of the big car companies though.
The main reasons 2-strokes became unpopular is because of oil pollution and noise. Add that to the fact that they were used for tiny cars and that really made them unfeasable in the US marketplace. They lasted longer in other countries, especially eastern-bloc countries.
They are generally inefficient. Increasing performance and efficiency tends to make them as expensive, heavy, and complex as 4 stroke engines. Each of the 2 strokes of the engine has to share 2 different functions that the 4 stroke does individually. The result is the limited RPM range of efficiency, loss of combustion gases before all power is derived, loss of fuel in the exhaust, and inefficient intake. The air intake system usually requires the air to be compressed twice to get it in fast enough. They are still popular with very small planes and hand held tools where the lower engine weight is significant. Also, two strokes produce a whining sound which is just not as cool as the rumble of a 4 stroke engine.
Really? How are they preventing unburned fuel from going out the exhaust? I thought a 2-stroke by definition would open the intake and exhaust port at the same time which would let some intake charge escape out the exhaust.
Edit: Or maybe you could make a direct injection 2-stroke… wait with injecting the fuel until the piston is up far enough to block the exhaust port. That would be interesting.
That’s what was being done in the 2 stroke Chrysler was attemping. That engine also used a sealed crankcase so it didn’t need lubricating oil added to the fuel. But the engine did have to power a compressor for the air intake since it wasn’t compressing intake air in the crankcase. Many 2 strokes also use a baffle in the the exhaust pipe to produce a pressure wave which bounces the excess fuel and air in the exhaust back into the cylinder increasing the compression. But that’s another factor that limits the efficient operating range of the engine.
Really, the definition of a two-stroke is that it produces a power stroke every crankshaft revolution, instead of every other one like a four stroke. The two main types are the crankcase-scavenged ones like your typical chainsaw/leaf blower/R/C engine where the air fuel charge is drawn in through the crankcase, and the externally scavenged type like the Detroit Diesels and the Chrysler engine which use a blower to charge the cylinder with air and direct injection for fuel. The latter type can’t blw raw fuel out the exhaust since fuel injection occurs after the exhaust outlet is closed.
Mercury and Evinrude two stroke outboard boat engines do exactly this. They are crankcase scavenged, direct injection engines. They also have a separate oil injection system so it no longer gets burned with the fuel.
If the main limitations are lack of torque and narrow RPM range, does that make them suitable for a electric-drive hybrid? (Where the engine will only run in the most efficient RPM and torque range, as it’s only powering a generator).
Boat engine, but… Evinrude’s E-TEC was the first outboard engine technology to win the American Environmental Protection Agency 2004 Clean Air Excellence Award, which recognizes low emission levels.
Is a 2-stroke more efficient than a 4-stroke when the 2-stroke is running at its optimum RPM? If so, perhaps it would be useful for charging batteries in an otherwise electric vehicle? Doesn’t the Prius hybrid use a non-standard engine cycle?
no; it has a higher power to weight ratio, but thermodynamic efficiency is lower.
most hybrids (at least Toyota, Ford, and Nissan- I think Honda too) simulate an Atkinson cycle by delaying intake valve closure. It does help efficiency but at the expense of power; the 2.5 liter Ford engine gives up 20 hp when run in the “atkinson” cycle.
(I put Atkinson in quotes because a true Atkinson Cycle engine has a physically shorter compression stroke than power stroke.)
I think the some hybrids use Atkinson cycle engines, which can be configured as 2 stroke engines, but I think these are 4 strokes. The idea of the Atkinson is to have a longer power structure to get all of the power from the combustion as possible. So combustion gases push the piston down until they are almost at ambient air pressure. In order for this to work the air/fuel intake can only partially fill a cylinder since combustion will increase the volume of the air/fuel mix.
ETA: As jt78817 said more simply, the intake/compression stroke would be shorter than the power/combustion stroke
