2009 BMW 750i: Engine Walkaround
April 29, 2009
Once you remove the plastic breastplate covering our long-term 2009 BMW 750i's engine, it's easier to see what's going on in there. Click the image above for a larger version.
Basically, there are two of everything. The flow paths for the two cylinder banks are completely independent--the two flow streams do not converge until after the exhaust exits the turbos.
Green arrows denote the location of the two airboxes where panel air filters reside. There are two probe-type hot-wire mass flow sensors (red arrows).
Purple arrows show a peek at the compressor housing of each turbo. The hard metal line and hose that curls around each turbo are coolant lines--when you shut off the engine, the coolant in the turbo's center housing boils. This is intentional, and is called a thermal siphon--the water vapor then moves up the line and more coolant takes its place, thereby preventing the heat that is "soaking back" (from the exhaust manifolds and downpipes) from coking the oil in the bearings.
The blowoff valves are integral to the compressor housings and are actuated electronically rather than pneumatically.
Yellow arrows point to the oxygen sensors located just upstream of the close-coupled catalytic converters. The closer the cat is to the turbo, the more quickly the cat's substrate can heat up and become functional, which improves emissions. BMW mounted these cats as close as they feasibly could have.
As you might imagine, there's a lot of heat shielding in the engine's vee around the turbos. Exhaust manifolds are dual-walled "tube within a tube" arrangements--there's an air gap that surrounds each exhaust primary tube. This helps the cat "light off" quicker and reduces the transmitted heat load. Still, I'm curious how well the surrounding bits hold up over time.
Blue arrows point to the liquid-to-air intercoolers. These are mounted directly to the front of the engine via isolation mounts. This makes for a very tidy and modular package. Had BMW used air-to-air intercoolers instead, they would have been forced to package much larger-diameter plumbing to the nose of the car, and then back to the intake manifolds. Air-water coolers can be smaller, too, since water has a high specific heat (i.e. it requires a lot of energy to raise its temperature one degree).
What you can't see are the plastic intake manifolds mounted to the outside of the cylinder banks. Again, this approach (the "inside-out" V8) is easier to package since the intake manifolds are relatively small.
Click the jump to get a better idea of the airflow paths of this engine.
Fresh air enters the intake tracts from ducts in the nose of the car. Air travels through a couple of noise-reducing intake honkuses before making a U-turn through the air filter(s). This filtered air then curls around and enters each turbos' compressor.
The now-boosted air (pink) is also heated during the compression process. If you're an engine, pressure is good but heat is bad.
Enter the intercooler. Intercoolers are heat exchangers, and they chill the air by transferring its heat to the (in this case) water. The "water" is probably windshield washer fluid or similar. I'll check. In any case, it has its own cooling loop and another heat exhanger (a "low-temperature radiator") located in front of the engine's radiator.
You usually won't see liquid-air intercoolers on dedicated racing vehicles outside of drag racing. It's difficult to adequately cool off the liquid once you've dumped a lot of heat into it like during an endurance race, plus these systems add a lot of complexity and potentially catastrophic failure modes--you don't want water entering your engine's intake system, unless you like tacoed connecting rods and ventilated blocks.
For a street cars, which never see prolonged full-load operation, liquid-air works great. There's very little pressure drop in their coolers, and the liquid is (initially) more effective at cooling than is air.
Jason Kavanagh, Engineering Editor