Because a 2-stroke engine fires twice for every firing of a 4-stroke engine. This means that the 2-stroke engine has potentially up to twice the output of a 4-stroke engine of the same displacement. Because it does twice the work at the same rpm, it also makes does it with half the parasitic friction (all else being equal). Eg. when both types were available on the market, no 125cc 4-stroke motorcycle ever makes as much power and/or weigh as little as a 125cc 2-stroke bike.
The problem with 2-Stroke designs...
However, the 2-stroke engine has many traditional short comings, some of them utter show stoppers. To begin with 2-stroke engines usually burn a premix of lubricant oil and fuel. This is because they tend to use the crank case as a piston pump to push the intake charge into the combustion chamber when both the intake and exhaust ports are open. This means that instead of having lubrication oil in the crankcase, they must fill it with a fuel, air and oil mixture. This is a serious problem because with enough oil content in the mixture to lubricate the load bearing main bearings, journal bearings and wrist pins, the mixture will burn in a smoky manner and is guaranteed to fail modern emission standards. Despite this, lubrication is still poor compared to 4-stroke engines leading to 2-stroke motors wearing out twice to three times as quickly as 4-cycle engines. To make matters worse, because both the intake and exhaust ports must be open concurrently at some point, 2-stroke cycles must either exhale the exhaust gases incompletely or over aspirate the intake charge such at a portion of the fuel-air mixture escapes into the exhaust. The former leads to reduced power output from not having enough air to burn all the fuel in the mixture. The latter leads to wasted fuel going straight into the exhaust. Both further compromises hydrocarbon emissions and lead to reduced fuel efficiency. As if that is not enough, the use of tuned exhaust systems provide a back pressure pulse to help achieve a balance between the two aforementioned problems also leads to a very narrow rpm range where the engine is optimally powerful, efficient and clean running. Even when everything is perfect, at the ideal rpm and load range, because both ports are on the lower lower of the cylinder scavenging is never as complete as a 4-stroke engine due to dead spots on the upper part of the combustion chamber and some degree of inefficiency cannot be avoided. All it all, inferior fuel economy, lousy emissions, poor longevity and narrow power bands have condemned the traditional 2-stroke engine to garden blowers and RC models. In fact, in many countries 2-stroke engines are outright banned not just on cars, but motorcycles and Jet Skis alike.
Direct Injected SPOHV Engine Changes Everything
Here I am presenting a concept that changes everything while retaining the advantageous of a 2-stroke design. The engine uses an overhead valve and side exhaust ports. Fueling is by means of direct gasoline injection during the compression stroke. It uses a wet sump lubrication system for the main bearings, journal bearings, wrist pins and part of the cylinder walls. The crankcase is filled with oil like a 4-stroke engine and is not used to pump a fuel-air-oil charge into the combustion chamber. Instead aspiration is enabled by an external centrifugal supercharger. The engine operates on a hybrid 2-stroke / Miller Cycle in that the intake valve stays open during a good portion of upward travel of the piston after the exhaust ports have closed. This results in an air charge that is above atmospheric pressure when the intake valve closes making this a true force induction engine. It also creates an asymmetrical compression and power stroke with the latter being longer than the former for superior combustion efficiency very much like Atkinson and Miller Cycle 4-stroke engines.
Emissions: Because the engine does not burn oil and there is no fuel introduced into the engine by the direct injectors until after the exhaust ports have closed there is no possibility of unburnt fuel escaping into the exhaust or significant amounts of oil being combusted. Because of this emissions are squeaky clean.
Longevity: Because all the elements in the bottom end is lubricated in the same manner as a 4-Stroke engine, durability of the SPOHV design should be comparable to contemporary 4-stroke engines.
Performance: With twice as many firings at any given rpm, the SPOHV engine is potentially twice as powerful as a 4-stroke engine of equal displacement. This is further enhanced by the fact that because the heads only have to house an intake valve and no exhaust valve, whereas the side ports on the cylinder walls are exhaust only, the total area of the intake and exhaust openings are insanely high compared to both traditional 2-stroke engines and DOHC 4-valve 4-stroke designs. This allows the engine to be lighter and smaller than a 4-stroke engine of equivalent output. Potentially half the size.
Refinement: While the 2-stroke cycle does nothing to change the harmonics of any particular cylinder arrangement it does double the number of firing pulses at any given rpm. In this sense, a 4-cylinder 2-stroke engine has pulse intervals equivalent to an 8-cylinder engine.
Fuel Economy: For any given amount of power made in an engine of a given displacement, the SPOHV engine turns over at half the rpm of a 4-stroke engine. It also has only 1 intake valve and 1 camshaft compared to 4 and 2 respectively for a DOHC 4-valve engine. This equates to significantly lower parasitic frictional losses. With extremely high intake & exhaust cross sections, the engine also has lower aspirational losses.
Forced Induction is a Must: Because the crank case is not used to push the intake charge into the the cylinders, an external compressor is a requirement. This can be a roots blower, centrifugal compressor, Lysholm Screw, G-ladder or even a piston pump. But some kind of air mover is needed and it cannot be a turbocharger because at idle with no load, the turbos will make no boost. Because the engine does not draw air into the cylinders with a downward stroke of the pistons and the engine won't run at all unless it is fed by a positive displacement air pump
Reduced Maximum RPMs: Because the engine has longer, heavier pistons as well as bigger heavier valves, the rpm limit is consequently lower.
Dual Exhaust Outlets: For optimal egress of exhaust gases the exhaust ports are on both sides of the engine block. This complicates exhaust routing a little, in the same manner that a V type engine does.
Hypothetical 2.0 liter 4-cylinder SPOHV engine statistics:-
Type: Direct Injected 2-Stroke, 4-Cylinder Engine
Valvetrain: Single Overhead Cam, 1-valve per cylinder
Construction: Aluminum Block & Heads
Aspiration: Centrifugal Blower Assisted, 2-stroke Miller Cycle
Bore x Stroke: 89 x 80.3 mm
Displacement: 1998 cc
Fuel Type: 87 Octane Unleaded Gasoline
Lubrication: Mobil 1 0W-20 Synthetic motor oil; 5 Quarts / 10,000 mile change interval
Power Output: 300 bhp @ 5200 rpm
Torque Output: 330 lb-ft @ 4200 rpm
Maximum Engine Speed: 5500 rpm
Edited by dwightlooi, 25 September 2011 - 01:37 AM.