Liq Cooling article

In  the  mid-1980s,  Teledyne Continental  Motors  initiated design 
of a 9.O-L, six-cylinder, liquid-cooled aircraft engine, with the 
goals of attaining higher specific output, lower fuel consumption1 
longer periods be- tween engine overhauls, and no weight penalty 
when compared with traditional air-cooled aircraft piston engines. 
Fol- lowing FAA certification in 1988, production of the engine, 
shown in Figure 1, started in 1989.


	To achieve an engine installation weight comparable with 
existing air- cooled aircraft piston engine installa- tions, cooling 
system features include in- creased coolant operating temperatures 
and extensive use of light alloys (in areas such as the cylinder 
head, coolant-pump housing and impeller, radiator, coolant 
circulation piping, and coolant tank). The engine cylinder was 
designed for 121 C maximum engine-coolant inlet temperature. The 
increased operating temperature reduces the heat rejection to the 
coolant (see Pigure 2) and permits a smaller radiator. The smaller 
radiator can also result in reduced aircraft drag. For operating at 
these elevated tempera- tures, higher system pressures were 
specified to suppress boiling and cavita- tion.

	The engine is designed with separate cylinder 
construction~ach cylinder as- sembly consists of a steel cylinder 
barrel and cast-aluminum cylinder head. The barrel and head are 
joined via a threaded joint in each part. The combustion cham- ber 
is formed in the cylinder head. A coolant jacket surrounding the 
combus- tion chamber is cast as an integral part of the cylinder 
head. An ethylene glycoll water mixture circulated through the 
coolant jacket plus oil cooling provides the he~ transfer from the 
combustion section.

	The inlet coolant is directed toward the valve bridge, spark 
plug, and exhaust port areas. The coolant then flows t~ ward the 
lower portion of the cylinder head and exits the outlet port. Oil 
spray from a crankcase-mounted nozzle aimed at the piston underside 
provides addi- tional heat transfer.

	The pump uses a cast-aluminum, closed-impeller design, 
operating at speeds up to 8100 rpm. A pressure rise of 207 kPa at a 
flow rate of 114 L/min. oc- curs at rated engine speed.

	The coolant distribution system works in parallel. From the 
coolant- pump exit, the coolant is divided into two branches and 
routed to each bank of three cylinders. It is equally divided among 
the cylinders by an orifice at each cylinder's exit. From each 
cylinder bank, the coolant is collected and routed to a 4.9-L 
coolant tank.

	An existing commercial, ground-ve- hicle ethylene glycol 
formulation was initially selected as the coolant. A 60/40% 
solution of ethylene glycol/distilled water was specified to ensure 
adequate freeze protection. Typical inhibitor pack- ages for these 
coolants include phos- phates, borates, hydroxides, nitrates, sili- 
cates, and triazole. Initial coolant change intervals at 250 h of 
operation or one year were selected, based on coolant supplier 
recommendations.

	To operate at the elevated coolant temperatures, higher 
system pressures are employed for suppression of boiling and 
cavitation. During the engine dyna- mometer development phase, a 
cylinder- head temperature profile was obtained to evaluate 
temperature gradients by em- bedding thermocouples throughout the 
cylinder head. The maximum metal tem- perature measured at the 
combustion chamber surface is 210 C between the valves. The typical 
coolant temperature rise across each cylinder is 8-11 C.


	No unusual cooling system problems were encountered, except 
for light coat- ings of a white substance that were ob- served 
intermittently in the engine cylin- ders and other components. These 
depos- its were analyzed using energy dispersant spectroscopy (EDS) 
and determined to contain significant amounts of phospho- rus and 
silicon.

	Engineers converted a twin-engine air- craft which was 
originally powered by air- cooled engines (see Figure 3) to a water- 
cooled system. The cooling system is con- figured to fit within the 
existing aircraft nacelle, as shown in Figure 4. New cowl- ings are 
used to manage airflow through the radiator and engine compartment. 
The coolant is routed through the system from engine exit to coolant 
tank, coolant tank to radiator inlet or thermostat~ontrolled by- 
pass, and radiator exit to coolant pump inlet.

	The coolant tank provides additional coolant storage, 
coolant/air separation ca- pability, system pressure control, coolant 
~vel monitoring, and supply of bleed cool- ant to the pump inlet. 
The coolant radiator is an aluminum plat~and-fin~ type unit with 
headers welded on each end. The coolant passage cross section is 
approxi- mately 1.5 x 2 mm, with a length of 585 mm. Except for 
flexible lines at the engin~ to-airframe connection points, all 
coolant is routed through components of alumi- num alloy material 
with Oring seals at each joint.

	The system's initial service experience was satisfactory. 
However, after 200-250 h of aircraft operation, excessive coolant 
temperatures (greater than 120 C) were reported. Inspection of 
several cooling systems revealed two distinct types of de- posits. 
Researchers observed a very light coating of a white substance on 
most sur- faces throughout the system, as well as a heavy buildup of 
a gel-type, off-white sub- stance at the exit side of the radiator 
(see Figure 5). The light coating was observed intermittently during 
development, but did not present a problem. Several clean- ing 
methods were tried without disassem- bling the radiator, but none 
was com pletely successful. Many of the radiators required 
replacement during this period.

	Researchers collected deposits from several radiators and 
subjected them to an elemental analysis using EDS tech- niques. 
Major elements identified in- cluded phosphorus, silicon, aluminum, 
and sodium. Inspection of the cooling system components did not 
provide an obvious source of aluminum material loss.

	Research into other liquid-cooled en- gines manufactured 
with aluminum heat-rejecting surfaces revealed similar deposits. 
Previous testing of these en- gines seemed to indicate that some 
ethyl- ene glycol formulations may not provide adequate corrosion 
protection, particu- larly at temperatures above 107 C.

	Possible solutions included operat- ing-temperature 
reductions, coolant-sys- tem design changes, and evaluation of other 
coolants. Reduction of operating temperatures or coolant-system 
design changes were considered undesirable based on the effort 
required for system development and certification. Other coolant 
candidates considered were pro- pylene glycol and ethylene glycol 
with a revised inhibitor package. Although propylene glycol is 
readily available, ex- perience with inhibitors for corrosion 
protection has not been well established. Based on a coolant 
supplier recommen- dation, an evaluation of a newly intr~ duced 
ethylene glycol with a revised in- hibitor formulation was selected 
as the favored approach.

	Earlier analysis of the deposits in the radiator suggested 
that some of the com- pounds in the initial coolant formulation were 
contributing to the excessive cool- ant-system deposits. The revised 
coolant included an inhibitor package relying on low silicate 
concentrations (250 ppm) plus specific carboxylic acid combina- 
tions for aluminum protection. The for- mulation is phosphate-, 
nitrate-, and amine-free.


	Initial evaluation of the new coolant was conducted on an 
engine dynamom- eter. No undesirable engine performance or coolant 
system deterioration was identified.

	While the new coolant was evaluated, the previously plugged 
radiators were examined to determine if they could be restored 
without removing the header tanks. Engineers fabricated a test rig to 
circulate hot solutions of several com- mercially available 
coolant-system cleaning compounds through the radia- tors. No 
successful cleaning method was identified for radiators with 
significant plugging. Cleaning of several aircraft coolant systems 
was also attempted, with similar results.

	The coolant was introduced to the air- craft fleet, using 
the same procedures for mixing, installation, and coolant-system 
operation as for the original coolant. Many of the radiators 
required replace- ment when the new coolant was intro- duced, since 
the coolant system could not be adequately deaned.

	Researchers initiated a coolant monitoring program 
lo?'several of the aircraft. The program consisted of withdrawing 
coolant samples at regular intervals, and conducting analyses (see 
Figure 6) and inspections of the coolant system.

	The analysis revealed a significant decrease in silicate 
levels between the imtial fill and the first sample interval, which 
was also reported in previous studies. Sufficient aluminum protection 
is indicated based on only trace amounts of aluminum with stable 
alkalinity, pH, and carboxylic acid concentration. The quantity of 
deposits in the cooling sys- tem was significantly reduced.

	Based on the favorable test results, researchers plan to 
continue monitoring the coolant performance with a sched- uled 
sampling program. By also evaluat- ing extension of the coolant 
change inter- val, and examining the performance of silicate-free 
ethylene glycol formulations containing carboxylic acid combinations, 
they hope to maximize the performance of cooling systems for 
aircraft liquid- cooled piston engines.	



Information for this article was provided by J. G. Wheelock, 
Teledyne Continental Motors Aircraft Products, and J. M. Burns, 
Texaco Chemical Company.



Reader Feedback

Please circle all the numbers on the enclosed

Reader Sen'ice Card which pertain to this
article.

Circle 181	This article is directlv related to
mv work
Circle 182	Technicallv, a verv useful article
Circle 183	Read for information only
Circle 184	Would like more articles in this
area
Circle 185	~d not find articlQ usef~