Friday, 1 February 2013

Exhaust

An important part of the M5 is its exhaust system. In the M5, the exhaust is particularly massive to support the large volume of air passing through the cylinders due to the extra turbo boost and high engine RPM's.

For example, my old E60 545i has a 4.4L engine, and it redlines at 6500 RPM. That means it is pumping 4.4*6500 L/min or about 475 L/sec. In my new F10 M5, also 4.4L, but the air volume is about 1.9x more due to the turbo boost up to 1.5 bar gauge, which really makes it an 8.5 L engine with a redline of 7200 RPM. Therefore the amount of air passing through is about 1000 L/sec. Imagine a big cube of air, 1 meter in each dimension, passing through the engine and out the exhaust each and every second.

The exhaust system is therefore critical in assuring this much exhaust gas goes out unhampered, without creating back pressure that robs power from the pistons due to pumping losses (see my blog article Engine Thermodynamics for an explanation of pumping losses).

The exhaust system starts when the exhaust valves open. Normally the problems start on a V8 engine at the exhaust manifold, the pipes that lead from the valves to the exhaust. Because the cylinder firing order is uneven (see my blog entry V8 Crankshaft to understand why), this causes pulsing in the pipes that backs things up. However, as explained in Engine - Turbos, the exhaust valves are on the inside of the V rather than the usual placement on the outside, and the cylinders are crossed over right in the exhaust manifold before the exhaust even hits the turbos.

This causes two smooth streams of exhaust to enter and therefore exit the turbos. The smooth flow eliminates the traditional issues with V8 exhausts.

After the turbos, the exhaust system continues at the two large catalytic converters which are the large golden "tanks" towards the rear of the engine.

There is an oxygen sensor plugged in at the top of each, and another sensor nearer the bottom side, used by the DME computers to optimize combustion and minimize exhaust gasses. The catalytic converter is essentially required by law in every vehicle as the only way to effectively meet the Califronia LEV II emissions standards. Here is a view of one of the catalytic converters removed.


The man is pointing at the end that attaches directly to the exhaust outlet of the corresponding turbo. The function of the cat is to act as a chemical catalyst for a wonderful set of three chemical reactions that converts some of the nastier by-products of combustion, namely Nitrogen Oxydes, Carbon Monoxide, and Hydro Carbons, into normal components of air (water vapour, nitrogen, and carbon dioxide). A catalyst is a chemical that plays an essential helping hand in a chemical reaction, but doesn't get used up in the process. Which is a good thing, because the catalytic materials are platinum, paladium, and rhodium, all very expensive rare earth metals!

The cat only starts working once it gets hot, so the placement of the cat up above the engine in the V is ideal in letting it get up to temperature quickly.

Emissions meet the California LEV II standards which for the first 50,000 miles limit carbon monoxide (CO) to 3.4 grams per mile driven, nitorgen oxides (NOx) to 0.05g, formaldehyde (HCHO) to 0.015g, non-methane organic gases (NMOG) to 0.075g, and particulate matter (PM) to 0.01g. Due to the turbo-charged nature of the car, it was not able hit ULEV standards (roughly 1/2 of LEV except for CO which is the same). This, unfortunately, puts the car into the bottom 18% of vehicles sold in the US, however is on par with my existing E60 545i.

After the cat, the exhaust flows under the car through generously sized pipes that bring the exhaust towards the rear.

The silvered material is heat shielding, necessary because of how hot the exhaust in this car can get (over 1000 C). Here we also see the first stage mufflers.


Here is the rest of the system to the rear mufflers, and then the rear mufflers themselves,


with the tailpipes exiting the car in four 3" pipes!


2 comments:

  1. The dual tail pipes exhausts look awesome on the BMW. Thank you for taking us through this. I didn't know that the silvered material is heat shielding, since the system always overheats due to the heated gases. The bends are also not sharp so there is smooth flow of gases. Great job!

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    1. Thanks. Good point about the bends and the smooth flow. It's a thing of beauty for sure!

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