Engine Cooling Ideas

This blog covers the background behind air cooling design

Overview
With the aircraft advancing and beign at home for a few weeks over Christmas it was decided to firm up the cooling of the cowl.

Why?

Simple, I would like to close up the hole under the nose gear but this is currently the primary means of cooling and engine air other components will be required.

The radiator is the primary concern as it size is fixed and options are few. Let get this straight it has worked in the factory aircraft for 300 plus hours and there are others on the way but one effect of closing a cowl is the internal temperatures climb and air has to have a routed in/out.

The first blog simply says you need ducts so that easy just build an internal cowl. The original design showed a connection to the fake exhaust - nice but not practical as to look correct they have to reside on the fuselage and the connection would be a bear.

What was needed was a means of extracting the air while maintaining the cowls look and this restricted the ideas to one - louvers. After a lot of work I have settled on a set of automotive louvers 5'' x 6.5 '' overall for a WRX.

Proposed duct - 550 mm overall

The inlet in light orange will be installed as the airflow is underrated in climb remembering that engine and oil cooling have to be handled by the radiator and the calcs show the radiator and water flow underrated this mode. The heat exchanger for the oil will be incorporated into the engine shroud for a number of reasons but one is to allow the exchanger to have an air blast to help offset load on the radiator.

The front duct will decelerate the incoming air raising the dynamic pressure with the duct on the reverse side accelerating the air reversing the process and then raising the pressure at the exit again. 

A wicker is just an additional trip at the front vent
The airflow behind the vent would be interesting

The other reason is that this mimics the pressure under the bonnet allowing air to exit into the moving stream under the cowl. The vents shown have a adjustable lips that extends up into the air flow effectively tripping the air stream creating a low pressure zone allowing the air in the duct to efficiently enter the moving stream of air. One [1] vent will be installed on the starboard side matching the SCATT hose.

RV12 Radiator / oil cooler duct

Another vent will be placed on the port side to vent 12 kw of heat created by the cylinders and radiant heat from the exhausts, radiator and other items. 

Note: This will only be effective with a sealed cowl


The fins are cooled by a downdraft pendulum used on the RV12, this will be covered in a later blog but in the heat exchanger plates located on its underside will be installed into the shroud inlet using incoming air to cool the cylinders taking some load off the radiator as the 64 mm diameter inlet has an excess of air available.

If door are installed they will have two sets of louvers fitted to allow the radiated heat to be ejected on the underbelly [see note above]

The final item examined was the supercharger air supply as this is also part of this equation. A 75 mm od supply was used for a simple reason, its what is available for the aftermarket automotive market. The inlet and filter are both carbon fibre and very light, cost effective with all connection being achieved using 75 / 64 mm od SCATT hose.

Comment

The rough estimate of the workload outlined below shows that there is a potential load at full power of 48 kw on the radiator with the water pump providing 50 Kw of water with 65 Kw of air available in level flight. If correct it means the system has to be very efficient in climb and when full power is beign used as every cubic cm of air must be harnessed along with the cooling water and there is a need to verify the capacity of the radiator in the real world carefully!. 

Wishing I had paid a lot more attention when in sitting in the classes - always wise after the event so below is some rusty thermodynamics.

The initials calcs were based on a article for the Europa by Jans
Apologize for any error in advance

Maximum power output Rotax Supercharged - 100 kW

Consumption at maximum power 40 l/h, i.e. 11.1 ml/s.

Gasoline represents 35 kJ of energy per ml.

Power consumption is therefore 11.1 x 35 = 388 kW

Efficiency is 100 / 388 = 26%.

Maximum cruise power output = 79 kW

Consumption at maximum cruise power is 29 l/h, i.e. 8.0 ml/s.

Power consumption is 280 kW

Efficiency is 28%.

75% cruise power output  55.1 kW

Consumption at 75% cruise power is 20.4 l/h, i.e. 5.67 ml/s.

Power consumption is 198 kW

Efficiency is 27.8%.

Total heat production to be removed estimated :

388 – (85 * 1.13) – (12.5 * 1.13) = 278 kW.

Assumptions

Rotax 914 heat removal prescription for maximum power operation (86 kW)

has to reject 45 kW as follows:

30 kW through cooling radiator

9 kW through oil radiator

6 kW from cylinder barrel fins

Supercharger factor = 912S/914 = 105/86 = 1.22

Under the cowling.

As the engine and exhaust system heat up they radiate more heat.

Stefan-Boltzmann says: about 5 / 10^11 x T^4 kW/m^2, T in Kelvin.

Table - T in Kelvin ('F) and corresponding radiation flux in kW/m^2 - :

700 (800'F) 12.0

800 (980'F) 20.5 - supercharger runs about 200 degrees cooler than turbo

900 (1160'F) 32.8

1000 (1340'F) 50.0 - turbo calcs

1100 (1520'F) 73.2

Estimate of 800K area (4 primary exhaust pipes): 0.10 m2

Radiated heat: 2 kW

Estimate of 800K area (muffler, turbo): 0.15 m2

Radiated heat: 3 kW

Say another 5 kW to be removed from under the cowling.

Total (39 * 1.22) + 5  = 53 kW.

Estimated exhaust gas energy 278 – 53= 225 Kw - that why a turbo is best for raw power

Assumptions

Similar efficiency for direct and indirect - via liquid - air cooling:

Inlet areas :

Cooling inlet radiator 210 cm^2 (estimated)

Cylinder barrel fins 20 cm^2 (estimate on diameter 5.0 cm)

Gear door louvers 18 * 7.5 * 1 = 135 cm^2 (18 louvers total)

11 ml gasoline per second with mass 11 * 0.7 g/s = 8.7 g/s

requires about 14 * 8.7 = 121 g/s of air per second

Assumptions

Air velocity into ducts is 40% of aircraft velocity

Specific Gravity 1.25kg/m3 at sea level pressure

Climb best rate = 120 km/hr (33 m/s) worst case air flow

Inlet radiator duct 210 cm/sq

Inlet supercharger 44 cm/sq

Specific heat capacity air = 1005 Kj/kg/deg K

Specific heat capacity water – 4.2 Kj/kg/deg K

Flow water = 73 l/min or 1.2 l/sec

Calculations:
Air flow supercharger

Using Q = (a*v*0.4) * s.g. = g/sec

44 * 33 * 0.4 * 1.25 = 726 g/s

NACA 3'' od Duct [44 sq cms]  will supply enough air by a 6:1 margin

Air flow radiator inlet

210 * (33 * 0.4) = 2370 l/s climb

210 * (60 * 0.4) = 5040 l/s cruse

Assume required radiator delta t = 10 deg /c (283 deg /K)

The Specific Heat formula is:  c = ΔQ / (m × ΔT)

Where:
  c: Specific Heat , in J/(kg.K)
  ΔQ: Heat required for the temperature change, in J
  ΔT: Temperature change, in K
  m: Mass of the object, in kg

Theoretical capacity air

1.005 = Q/ (2.4 * 1.25) * 10

      Q = 30 KW climb

dT = (Q / (Heat output turbo x Supercharger factor)) * target radiator
     = (30 / 39 x 1.22) * 10 = 6.2 deg C

1.005 = Q/ (5.0 x 1.25) * 10

      Q = 63 kW cruse

dTmax = 63 / 48 x 10 = 13.1 deg C

Theoretical capacity water

4.2 = Q/ (1.2 * 1) * 10  
Q = 50 kW