In a previous post I gave a simplified overview of turbocharging and why it includes a charge air cooler (CAC). I want to share a few more words about CACs. A casual look may lead you to believe it’s the same as an engine coolant radiator except air flows through it. This is partially right, but there are key differences.
CACs are designed and constructed to operate under very harsh conditions. Engine coolant radiators have it fairly easy by comparison. Hot coolant – around 180 – 190 degrees – starts flowing through the coolant radiator when the thermostat opens. It’s pressurized to about 15 psi. The pressure and temperature stay fairly constant until the engine is shut down and it slowly cools.
A CAC is subject to a wide variation in pressure – anything from atmospheric to 30 or 40 psi above atmospheric. The pressure changes almost instantaneously, depending on engine load. As hot air enters the inlet side of the CAC and travels across the core, it cools and the pressure drops slightly. The inlet tank, core diameters and outlet tank are sized to try to maintain even flow and pressures throughout the CAC. The temperature of the air flowing through the CAC also varies widely during normal operation and the CAC cools very quickly when the engine is shut down.
These pressure and temperature variations in the CAC require careful design and robust construction – something much more durable than an engine coolant radiator. Where coolant radiators usually have the inlet and outlet tanks formed from sheet metal, CACs are usually made from castings. They can be die cast or sand cast. This is a much more expensive process than fabricating from sheet metal.
CACs come in a variety of shapes and sizes. Engine manufacturers such as Cummins or Caterpillar don’t normally include the CAC when they supply engines to vehicle manufacturers – they supply a specification that must be met and it’s up to vehicle manufacturer to come up with the CAC. On heavy duty trucks like a Peterbilt or Kenworth they use their own standard designs whenever possible to take advantage of economy of scale.
RV manufacturers may use an off-the-shelf part or they might have a specific size and shape CAC made exclusively for a particular coach. Either way, a CAC for a motorhome is much more expensive than one for a heavy duty truck. They don’t come cheap!
Last Thursday when the guys at Albuquerque Rocky Mountain Cummins pressurized our CAC and showed me where it was leaking, I had a sinking feeling. If a seam in the core is leaking, it can usually be repaired by welding it. If the cast tanks are cracked and leaking, it’s not usually repairable. The guys at the shop seemed confident that they could get our CAC repaired. I was skeptical.
On our coach – and most coaches with side radiators – the heat exchangers are combined in a stack. The engine coolant radiator is mounted closest to the engine compartment with the CAC – which is dimensionally similar – mounted in front of it, nearest the outside of the coach. Stacked on top of that is a smaller heat exchanger to cool the hydraulic fluid that operates the fan motor. I had the entire stack sent to a radiator specialist to re-core the radiator, repair the CAC and clean (rod out) the hydraulic fluid cooler.
Yesterday the radiator shop confirmed my fears. Our CAC is toast – they cannot repair it. We’ll have to replace this very expensive component and we’ll have to wait for them to acquire the proper part. They’re saying they can have one within a week. I’m hoping so. If not, we’re in trouble – it’s not just time and money. The Albuquerque International Balloon Fiesta begins at the end of next week. Starting Wednesday night, all of the hotels in the area are fully booked – no vacancies!
This morning Donna got on the 7am shuttle to the airport. She’ll be in Bennington, Vermont visiting her parents for the next week. With any luck, she’ll be back in time for us to take the repaired coach to the Balloon Fiesta Park!