01f25073
My question: Is the same true for the diesel 2-stroke vs. 4-stroke engines (generally speaking)? I am not talking about the emission issues, just the performance and maintenance issues. Thanks for your replies!
I think i've got this "quote thing worked out :-?
Where do we start...what LMJD says is probably conservative...the only similarity is they fire on every stroke...and that in its self delivers the power.
On the dirt bike front...2-strokes will always deliver more torque and hp all things being equal...HOW CAN I SAY THIS...ever been interested in motorcycle trials...2-strokes ruled, and even Honda couldn't develop a 4-stroke to compete in a sport where torque is everything...rpm is only constrained by piston speed...except... in 2-strokes the piston has to interact with holes in the cylinders, called ports. This limits cylinder ported 2-strokes from having the ultra short strokes that the likes of F1 and Indy engines run, because of piston rock in the cylinder bore.
A problem for ALL 2-strokes is managing piston and ring temperature. Next to combustion detonation, this is the biggest issue causing seizing. Most of a pistons heat is transfered through the piston ring to the cylinder...Overheating rings typically on the exhaust port side burn off lubricating oil and begin to scuff...and pistons overheat, swell and themselves scuff and jam in the bores. That rattly "ring" 2-stroke bike & chainsaws make as they get older is piston slap...eventually piston slap causes the piston to lip out on the port and bang, you're off :-! I've seen it happen at idle!
We ALL know combustion efficiency is directly influenced by combustion chamber design, higher compression ratio (up to a fuels auto ignition) and intake charge swirl.
2-stroke crankcase scavenged engines of the type found in chainsaws, outboards and motorcycles, are more "efficient" in combustion than 4-strokes but less "efficient" in scavenging...HOW IS THIS!!...without playing with words... it depends on your interpretation of "efficient". In this case they burn the fuel/mix more effectively (quicker and more of the mix) than 4-strokes (Fact!) due to very compact and ideal shaped combustion chambers, no valves in the way dictating chamber design...squish band lands...or plug placement...also...2-strokes are more efficient at light load part throttle, as long as they're not 4-stroking (which indicates a richer than ideal mix for that engine state).
Fact! 2-stroke engines run at their designed C/R ALL THE TIME
Consider! What is the compression ratio of your car...when your trundling along the road...at 1/4 throttle. Think about it! if the enf=gines designed C/R is for arguements sake 10:1 uncorrected, and at 1/4 throttle the engines breathing in 1/4 its volume...that would be about 2.5:1...doesn't make for efficient combustion! and your car spends most of its life at this engine state.
Because a 2-stroke "can't" evacuate the cylinders volume of gas...any gas...it always traps a cylinders full volume...even at idle...a 4-stroke only traps what it sucks in because its expelled the cylinders corrected trapped volume. At idle some exhaust gas is sucked back into the cylinder before the exhaust valve closes, because there is a high vacuum on the piston side of the throttle...so the idle speed charge is affected by residual hot inert gases...
If efficient scavenging means, all the exhaust is expelled for a clean unpolluted! fresh intake charge...then 4-strokes are more efficient, ahh, that is...only at full throttle...because at part throttle, this condition is never met.
2-strokes are actually more efficient in scavenging...and a result of this is they can be tuned much leaner than 4-strokes. The principle of HCCI Homogenous Charge Compression Ignition is a factor of a 2-strokes lean burn capabilities...because with the fuel mix passing through the crank case, it absorbs heat and reaches a finer point of evaporation and higher volatility which enables better burn and leaner mix as a result. In the days of aircooled 2-stroke bikes (DT250) and you've just thrashed your bike down the beach...screeched to a stop and turned your key off...it would some times continue to run, and for some time...unless you clicked into gear and dropped the clutch...sometimes it would even run backwards...that made experts ask what was happening...HCCI...because the engine (piston & head) had reached a critical temperature and spontaneous combustion was happening...not necessarily detonation, though that could have been happening too.
We are running out of Crude oil...HCCI is a serious solution.
2-strokes of old could run for ever...they could compete performance wise with 4-strokes, with small ports, though they were typically made as budget machines due to their few parts (machining) and simple design...modern high performance 2-strokes have very large ports and typically have port bridges just a millimeter or two for pistons and rings to slide over...that is a problem for ring and cylinder life. Motorcycle trials bike engines seem to have quite long lives...I don't know the overhaul life cycle of modern MX 4-strokes. I do know they can't pump out the power of the 2-strokes though.
A Question to ponder! ever wondered why...when you're riding down the street and then button off...the engine goes ping......ping.....ping and does the same at idle. Play with the secondary jet mixture needle on a chainsaw and you can make it idle smooooth as...and firing on every revolution, unfortunately you can't get it to pick up rpm...think about it!
Thats not a problem of the engine or its design, more of the carburetor...
Boaties have been hailing the praise of direct injection being the halyluya of the 2-stroke outboard...sure they are hp/hp lighter and more fuel efficient...BUT... the problem of oil carry over with the total loss lubrication still remains a difficult emission to resolve...uniflow 2-strokes don't have this problem with the same lubrication systems as 4-stroke engines.
Uni-flow 2-stroke diesels are a way different beast and their principle functions have very different roles, their design criteria and operation are very different. Factor in the different design criteria and issues, like for like, of cylinder porting (Bristol Hercules & Centaurus sleeve valve 4-stroke radial Aero engines) vs poppet valves...look how long it took to develop reliable poppet valves...The problem Bristol encountered which halted development, was the turbojet, even though radial engines persisted unexpectedly in manufacture for another 30-40 years.
My Question; Why haven't the Detroit Diesels been refined...common rail injection, single overhead cam & rockers, integrated dry liners and modern porting concepts to the same level as modern 4-strokes...has it been corporate politics...definitely the Commer TS3 & TS4 opposed piston 2-stroke run head on into politics when Rootes got bought out in 1966/67 by Chrysler, they dumped the TS3 to prevent it competing with the new (yet to be realised, disastrous) 170hp Cummins Vale engine they'd just finished spending a huge sum developing ready for production! and was installed in Leylands new cab over 10 tonner.
A KEY difference between Spark ignition engines and diesel...is diesels don't, and can't run a throttle, because they have to run their designed compression ratio to promote compression ignition, so they're running full volumetric efficiency all the time and purging as near, all exhaust gases.
At low power settings (throttle) combustion is running in a very lean state (excess oxygen) however pumping losses can be high relative to throttled Spark ignition (Si) engines. A consequence of this is diesels need to have high charge swirl designed in, to promote effective fuel air mixing, at the moment of combustion, no pre-mixing as with your bike. Cylinder ported 2-stroke diesels are much more effective and efficient than poppet valved designs in the this regard.
Excess swirl transfers heat into surrounding cylinder walls, pistons and heads (blast effect or chill factor in reverse). A reason diesel pistons are built so heavy/solid yet still need oil cooling and or fire plate piston crowns and fire rings either in the piston or around the top of the cylinder.
But I've run out of time and gotta go for now