TROUBLESHOOTER

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Tur­bocharg­ers Steve Zim­mer­man

Mod­ern diesel en­gines rely on tur­bocharg­ers to max­i­mize per­for­mance and min­i­mize en­vi­ron­men­tal im­pact. While ma­jor in­ter­nal en­gine break­downs do hap­pen, it’s far more likely you will face the fail­ure of one of your pe­riph­er­als, like a wa­ter pump or fuel pumps, and this in­cludes tur­bocharg­ers. In fact, tur­bos are es­pe­cially prone to trou­ble, and the pru­dent cruiser should know where to look for signs of im­pend­ing prob­lems and how to min­i­mize the risk of fail­ure. Let’s be­gin by look­ing into the op­er­a­tion and com­po­si­tion of your turbo.

COM­PRESSED EN­ERGY

A diesel en­gine com­presses fuel and oxy­gen with enough pres­sure to cre­ate a con­trolled ex­plo­sion. The more en­ergy pro­duced in this re­ac­tion, the more power that can be de­liv­ered to your pro­peller. The vol­ume of fuel and oxy­gen is limited by the size of each cylin­der, so that means if you want more en­ergy, you must squeeze more BTUs into each mea­sure of fuel or more oxy­gen into each mea­sure of air.

As fuel heats up, it expands. This ex­pan­sion means a loss of en­ergy. To see how this works, imag­ine that on a cool morn­ing you see a con­tainer of diesel fuel with the level ex­actly at the one-gal­lon mark. If you pur­chase one gal­lon of fuel that morn­ing, you will have ac­quired 130,500 BTU of en­ergy. If you wait un­til later that af­ter­noon af­ter the con­tainer has been sit­ting in the sun, the fuel will have ex­panded and the level will now be above the one-gal­lon mark. If you buy ex­actly one gal­lon dur­ing the hottest part of the day, you will leave some BTUs in the con­tainer.

This same prin­ci­ple ap­plies to air. A cu­bic foot of air that heats up and expands will con­tain less oxy­gen (and there­fore less en­ergy) than cooler air. But what if we com­press the air? For ex­am­ple, 1.2 cu­bic feet of air com­pressed into a one-cu­bic-foot con­tainer will con­tain more oxy­gen—and this will im­prove en­gine per­for­mance. A tur­bocharger does just that—it forces air into the cylin­ders, com­press­ing it to in­crease the com­bus­tion and power. We can mea­sure the amount of air com­pres­sion by com­par­ing the pres­sure cre­ated to am­bi­ent at­mo­spheric pres­sure. We re­fer to this dif­fer­ence as “boost pres­sure” or “turbo boost.”

UN­DER­STAND­ING THE ME­CHAN­ICS

The cast hous­ing of your tur­bocharger con­tains the fol­low­ing main com­po­nents: a tur­bine wheel, a com­pres­sor wheel, and an axle to con­nect the two wheels. Oil from the en­gine lu­bri­cates th­ese parts, and coolant from the en­gine helps re­move heat from the hous­ing.

The tur­bocharger works much like a fan: Spinning blades ac­cel­er­ate the air­flow. Driven by the en­gine’s ex­haust gas, the tur­bine on one end of the axle spins at a re­mark­ably high rate (as high as 100,000 rpm is not un­com­mon). This high-speed ro­ta­tion also spins the com­pres­sor wheel, and the process forces com­pressed air into the en­gine.

HOT AIR

Con­vert­ing the waste en­ergy from the ex­haust gas into com­pressed air for the en­gine’s in­take cre­ates its own prob­lem,

how­ever. Com­press­ing the air causes it to heat up, and re­mem­ber that when a gas heats up, it con­tains less oxy­gen. This means less en­ergy is go­ing to the en­gine, which coun­ter­acts the ben­e­fit of the tur­bocharger. This is why the sys­tem also in­cludes some type of air cooler (ei­ther an in­ter­cooler or an af­ter cooler, de­pend­ing on the con­fig­u­ra­tion). This heat ex­changer is de­signed to re­duce the tem­per­a­ture of the com­pressed air be­fore it en­ters the cylin­ders. In most cases this ex­change of heat uses cir­cu­lat­ing sea­wa­ter to cool the air (see Pas­sage­Maker, April 2017).

To re­cap, ex­haust gas that is 800 de­grees Fahren­heit spins a tur­bine at some­thing like 100,000 rpm, re­ly­ing on a sys­tem of bear­ings and seals to min­i­mize fric­tion via lu­bri­cat­ing oil while an air cooler us­ing sea­wa­ter re­duces the tem­per­a­ture of the hot com­pressed air be­fore it en­ters the en­gine. What could go wrong? Plenty.

LU­BRI­CA­TION PROB­LEMS

Ac­cord­ing to BorgWarner Turbo Sys­tems, an anal­y­sis of tur­bocharg­ers re­moved from ser­vice re­vealed that 40% of failures could be traced to poor lu­bri­ca­tion. The turbo de­pends on lube oil cir­cu­lat­ing from the en­gine to min­i­mize fric­tion and to cool the bear­ings. Due to the ex­cep­tion­ally high speeds at which the tur­bine op­er­ates, even a mo­men­tary in­ter­rup­tion of oil flow can trash the bear­ings. So keep­ing up with the en­gine lu­bri­ca­tion re­quire­ments is key to pro­long­ing the life of your turbo. On older boats with un­known ser­vice his­to­ries, it can be help­ful to re­move, clean, and in­spect the small oil lines lead­ing to the turbo.

Lu­bri­ca­tion can also be an is­sue at startup or shut­down. For­tu­nately, on cruis­ing boats our en­gines have time to warm up as we un­tie lines and get un­der­way. Start­ing the en­gine gets oil flow­ing to the turbo, but flow alone does not suf­fice—the oil must be warmed to idle tem­per­a­tures be­fore load­ing up the turbo. Proper en­gine shut­down pro­ce­dures also must be fol­lowed. The old prac­tice of gun­ning the en­gine in neu­tral and shut­ting it down should never be done. The turbo will con­tinue to spin and wind down, but with the en­gine off, the oil flow stops, re­sult­ing in pre­ma­ture wear. In­stead, the en­gine should be al­lowed to cool down at idle for about five min­utes be­fore you hit the stop but­ton. This al­lows the cool­ing sys­tem to dis­si­pate heat from the turbo be­fore the oil flow stops. With­out time to cool down, the hot turbo will bake any resid­ual oil, cre­at­ing a buildup of harm­ful de­posits.

A sim­i­lar prob­lem can oc­cur at startup on en­gines that have been sit­ting for a few months with­out start­ing. In th­ese sit­u­a­tions, the oil will have drained out and any residue will have lost its lu­bric­ity. Start­ing the en­gine will cause the turbo to spin be­fore fresh lu­bri­cat­ing oil reaches the bear­ings, cre­at­ing un­nec­es­sary wear. While one in­stance would not be a prob­lem, bear­ing life would be sig­nif­i­cantly re­duced if this were a re­cur­ring is­sue. On some en­gines you can avoid this wear by briefly crank­ing the en­gine while en­gag­ing the stop Ex­haust gas (red) en­ters the cham­ber and spins the tur­bine on the right be­fore ex­it­ing into the ex­haust sys­tem. At the other end of the axle a com­pres­sor wheel then turns, forc­ing air into the en­gine at the top-left (blue). The blue on the ex­haust side (right) shows coolant cir­cu­la­tion from the en­gine.

switch. The crank­ing will force fresh oil into the turbo while the stop switch will pre­vent it from start­ing and spinning the turbo.

AIR IN­TAKE IS­SUES

Again, ac­cord­ing to the BorgWarner anal­y­sis, an­other 40% of the failures were caused by for­eign ma­te­rial en­ter­ing the tur­bine or the com­pres­sor. What’s for­eign ma­te­rial? Any­thing other than clean air. Air en­ters the tur­bocharger from two dif­fer­ent sources: ex­haust gas through the tur­bine and am­bi­ent en­gine room air

through the com­pres­sor wheel. The air fil­ter, well main­tained, will pro­tect the air in­take side, and for this rea­son dam­age to the com­pres­sor wheel hap­pens far less of­ten. Any metal that en­ters the ex­haust sys­tem (very small pieces from a fail­ing valve or a scuffed pis­ton, for ex­am­ple) will cause se­ri­ous dam­age to the thin, pre­cise vanes on the tur­bine. Bits of im­prop­erly fit­ted or im­prop­erly at­tached gas­kets or hoses can do the same. Given the ve­loc­ity of ro­ta­tion and the rel­a­tively frag­ile na­ture of the com­po­nents, it doesn’t take much to cause dam­age that can put your turbo out of com­mis­sion.

OTHER FAC­TORS

Ma­rine tur­bocharg­ers face an is­sue rarely seen in au­to­mo­tive and in­dus­trial ap­pli­ca­tions: con­tin­ual ex­po­sure to salt. The sea­wa­ter in­jec­tion into the ex­haust riser pro­duces a mist while op­er­at­ing, and when sit­ting at the dock the ex­haust sys­tem con­tin­ues to ex­pose the turbo to salt air. Over time, re­gard­less of en­gine hours, this ex­po­sure eats away at the tur­bocharger’s hous­ing. Pit­ting grad­u­ally in­creases the clear­ance be­tween the tur­bine fins and the hous­ing, re­sult­ing in a loss of boost pres­sure. When this con­di­tion de­vel­ops you will ex­pe­ri­ence re­duced en­gine per­for­mance and ex­ces­sive ex­haust smoke.

While man­u­fac­tur­ers en­gi­neer their tur­bocharg­ers to op­er­ate at high tem­per­a­tures, the units still have de­sign lim­its. A re­stric­tion in the air sup­ply into the en­gine, such as a fouled fil­ter, will drive up the en­gine and ex­haust gas tem­per­a­tures. Ex­cess heat can cause cracks in the hous­ing or fail­ure of the tur­bine.

Chron­i­cally run­ning your en­gine at low rpm for long pe­ri­ods can cause a va­ri­ety of en­gine prob­lems as well, in­clud­ing buildup

of car­bon on the tur­bine fins and in­side the hous­ing. Be­cause the tur­bine blades must pass within thou­sandths of an inch of the hous­ing so the com­pressed air can­not es­cape, the tur­bine and com­pres­sor wheels must be finely tuned and bal­anced when in­stalled. Any buildup on the blades can throw the wheel off bal­ance enough to cause vi­bra­tion and bear­ing wear, and pos­si­bly break­age of the blades. If you are see­ing ac­cu­mu­la­tion of soot around your ex­haust dis­charge, you prob­a­bly have ac­cu­mu­la­tion in­side the turbo.

You can avoid this sit­u­a­tion by pe­ri­od­i­cally (as part of each day’s run) run­ning your en­gine at loads close to 80% or even higher. A pro­ce­dure re­ferred to as “turbo wash­ing” uses a wa­ter­based sol­vent to clean the fins and should be per­formed by a skilled me­chanic. Some en­gine man­u­fac­tur­ers spec­ify a ser­vice in­ter­val based on en­gine hours for this pro­ce­dure.

IN­SPEC­TION

Guessing at ex­pected tur­bocharger longevity amounts to just that–guessing. In recre­ational ap­pli­ca­tions, 3,000 to 4,000 hours might be ex­pected. Of course, much higher in­ter­vals are achieved as well. Tur­bos live in an ex­treme en­vi­ron­ment and de­spite your best ef­forts, at some point your turbo will re­quire a re­build or re­place­ment. The steps out­lined above will ex­tend your turbo’s life and will pro­vide warn­ings be­fore un­ex­pected fail­ure.

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Up close and per­sonal view of a tur­bocharger’s com­pres­sor wheel.

On this tur­bocharger, ex­haust gas en­ters at the red area at the top and ex­its on the lower right at the mixer el­bow. The air fil­ter on the left al­lows fresh air into the com­pres­sor. The pipe run­ning un­der the air fil­ter di­rects the com­pressed air into the

A tur­bocharger grave­yard. Care­ful main­te­nance and some ba­sic en­gine op­er­a­tion pro­to­cols will help keep yours from end­ing up on th­ese shelves be­fore their time.

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