TDC

BUT MOSTLY NOT IN MO­TOR­CY­CLING

Cycle World - - Front Page - By Kevin Cameron

Dur­ing the 1970s some mo­tor­cy­clists wanted to make a moral is­sue of how many pis­ton strokes an en­gine needed to com­plete its power-pro­duc­ing cy­cle—four strokes good, two strokes evil. And many a rac­ing fan has re­ferred to the long pe­riod of two-stroke dom­i­nance as the “for­got­ten era.”

I don’t see this as a moral is­sue. Twostroke or four, an en­gine must carry out the same four es­sen­tial tasks of in­take, com­pres­sion, power, and ex­haust. A twostroke fits two of those into each of two pis­ton strokes, while a four-stroke pro­vides (roughly) a sep­a­rate pis­ton stroke for each task. Ei­ther way, it’s the same air and the same fuel; com­bus­tion of a com­pressed fuel-air mix­ture re­leases heat that drives its pres­sure very high. That pres­sure acts on a pis­ton to ro­tate a crankshaft. The rest is just house­keep­ing—sweep­ing out the ex­haust gas, bring­ing in the fresh charge. It’s like clean­ing up and re­stock­ing the kitchen af­ter a meal. The meal’s the thing!

Dur­ing WWII, Detroit Diesel pro­duced some 57,000 two-stroke diesels to power tanks and land­ing craft, and very sim­i­lar en­gines con­tinue to power heavy trucks and equip­ment to this day. Each cylin­der has four ex­haust valves in its head, and as its pis­ton ap­proaches BDC it ex­poses a ring of fresh-charge ports served by a large Roots ro­tary blower (the six-cylin­der 6-71 Detroit Diesel was the source for the alu­minum-cased blow­ers that orig­i­nally pro­vided su­per­charge air for Top Fuel drag cars). Pure air was blown into the cylin­der, chas­ing the depart­ing ex­haust gas up to the ex­haust valves. Com­pres­sion could be­gin once both the ex­haust valves and fresh-charge ports had closed and near TDC fuel was in­jected, ini­ti­at­ing com­bus­tion by the heat of com­pres­sion.

Fair­banks Morse in the US and Napier in Eng­land pro­duced two-stroke op­posed­con­trols pis­ton diesels for rail and sub­ma­rine use, and thou­sands of Soviat tanks also em­ployed such op­posed-pis­ton two-strokes. In each cylin­der there are two pis­tons, com­press­ing fresh air be­tween them. a ring of ex­haust ports and the other uncovers fresh-charge ports, so the fresh “scav­enge” air moves in one di­rec­tion—hence the name “uni­flow” for this en­gine type—from the fresh charge ports to the ex­haust ports. This sys­tem has the con­sid­er­able ad­van­tage of re­duced com­bus­tion heat loss be­cause it has no cylin­der heads.

The ob­vi­ous rea­son for de­vel­op­ing the two-stroke prin­ci­ple is that such en­gines fire twice while a four-stroke fires once. That is the rea­son nearly all of marine in­ter­na­tional trade is car­ried by ships pow­ered by large two-stroke diesels. The most ef­fi­cient marine pro­pel­lers are large slow-turn­ing ones, and be­cause re­duc­tion gear­ing is ex­pen­sive and heavy, there is strong mo­ti­va­tion to find a prime mover that can be di­rectly cou­pled to a 30-foot pro­pel­ler whose six blades re­quire more or less 100,000 hp to drive it at 85 rpm. In case you’re won­der­ing what torque that re­quires, it’s roughly 6 mil­lion pound-feet. Only a two-stroke could de­liver that within the re­quired en­gine vol­ume and weight.

The sim­ple crank­case-scav­enged and car­bu­reted two-strokes that many of us grew up with paid a high price for their light weight, low bulk, and sim­plic­ity. That price came in the forms of heavy fuel con­sump­tion (at least 30 per­cent greater than that of a four-stroke of equal power) and very high ex­haust emis­sions of un­burned hy­dro­car­bons (UHC). Here’s why. As the pis­ton rises, some form of valve opens to ad­mit fuel-air mix­ture from a car­bu­re­tor into the crank­case. As the pis­ton falls, it slightly com­presses this mix­ture in the case. Mean­while, the cylin­der has fired just be­fore TDC and the re­sult­ing pres­sure forcibly

presses the pis­ton down. By the time the pis­ton has de­scended by half its stroke, it has taken about 85 per­cent of the us­able pres­sure en­ergy in the com­bus­tion gas, so the pis­ton be­gins to ex­pose one or more ex­haust ports cut through the cylin­der wall. Ex­haust rushes out, and if a prop­erly shaped ex­haust pipe is present, a first re­turn wave of low pres­sure in­vites any re­main­ing ex­haust gas to leave. At three-quar­ter stroke, two or more fresh-charge or “transfer” ports are ex­posed by the pis­ton. Th­ese, fed from the com­pressed mix­ture in the crank­case, form jets en­ter­ing the cylin­der on a loop­ing path. Dur­ing this process, some fresh mix­ture short-cir­cuits to the open ex­haust port, caus­ing the high fuel con­sump­tion and UHC. The ris­ing pis­ton closes the transfer ports first, and as the ex­haust is about to close, a late wave of high pres­sure ar­rives from the ex­haust pipe, stuff­ing back into the cylin­der the lead­ing edge of the freshcharge loop, pre­vent­ing its loss.

Just as na­tional emis­sions agen­cies were nix­ing fur­ther pro­duc­tion of two-stroke road bikes in the 1980s, a new tech­nol­ogy of di­rect fuel in­jec­tion (DFI) was emerg­ing. DFI crank­case-scav­enged en­gines drew in pure air, and pure air jetted into the cylin­der from the transfer ports. Then as soon as the ex­haust port had closed (roughly 90 BTDC), fuel in the form of ex­tremely small, fast-evap­o­rat­ing droplets (10 to 40 mi­crons di­am­e­ter) was rapidly in­jected. This was hard to do be­cause with ig­ni­tion oc­cur­ring at about 20 BTDC, there are only 70 crank de­grees in which to in­ject and evap­o­rate the fuel (hence the need for droplets much smaller than those pro­duced by four-stroke-type port in­jec­tors).

In a vari­ant of this process, some Bom­bardier snow­mo­bile en­gines in­ject their fuel into the transfer flows, far enough

THE SIM­PLE CRANKCASESCAVENGED AND CAR­BU­RETED TWOSTROKES THAT MANY OF US GREW UP WITH PAID A HIGH PRICE FOR THEIR LIGHT WEIGHT, LOW BULK, AND SIM­PLIC­ITY.

up­stream and timed such that al­most none can reach the ex­haust port be­fore it closes.

Back in the 1980s DFI seemed com­plex and ex­pen­sive, so it was not widely adopted. Since then, four-stroke in­jec­tion—es­pe­cially the “GDI” form that in­jects di­rectly into the com­bus­tion cham­ber—has be­come equally com­plex and ex­pen­sive. So, no, we can­not re­turn to the days of sim­plic­ity and low cost, but, yes, the ad­van­tages of the two-stroke prin­ci­ple re­main at­trac­tive be­cause emis­sion­scom­pli­ant two-strokes can be built. In snow­mo­biles and per­sonal wa­ter­craft, low-emis­sions GDI two-strokes re­main eco­nom­i­cally im­por­tant. There are snow­mo­biles pow­ered by large four-strokes, but re­view­ers coyly term them “suit­able for groomed trail op­er­a­tion.” That means, if they get stuck in deep pow­der, you won’t en­joy drag­ging those heavy things out. Twostrokes still have their uses!

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