Stock Car Science

The stock car science blog has been a little behind due to a really big project I’ve been working on that has taken up every spare moment of writing time the last few months. The good news is that there (finally) will be an announcement at Texas Motor Speedway the weekend of the April race. More as it develops.

The Las Vegas race, like last week’s California race, had more than its share of blown engines. The rise in the engine fatality rate is being attributed to a combination of factors. One is that the tires are a little grippier, which allows higher engine speeds. The second is that the temperatures have been cooler. Air density increases as the temperature drops. If you compare two equal volumes of air at different temperatures, the cooler volume will have more air molecules. The larger number of air molecules per volume increases power, but that also creates an increase in heat.

Four out of five Roush engines gave up at Las Vegas. Jack Roush noted that the team had chosen to use the higher ratio of the two allowed rear-end gears (3.89), which increased the engine speed by about 200 rpm. Qualifying runs were coming in around 9900 rpm. In addition to being grippier, the tires didn’t fall off as much as expected, so the speed stayed elevated for a longer time. Hendrick Motorsports’ problems at California were attributed to “valve train failures that were related to a specific batch of parts from a vendor".

This is the second week Toyota cars experienced engine problems. Last week at California, Brian Vickers won the pole, but was forced to start in the rear after discovering an engine problem post-qualifying. Michael Waltrip’s engine didn’t even let him make a qualifying attempt.

Vickers qualified 21st this week at California, but he, teammate Scott Speed and Michael Waltrip Racing’s David Reutimann (qualified 4th) and Marcos Ambrose (qualified 5th) all had to change engines.

Kyle Busch (who won the pole and ultimately the race) had to change his engine; however,Joe Gibbs Racing has their own engine program and Busch’s engine issues are unrelated to the other Toyota’s.

The engines for Red Bull Racing and Michael Waltrip Racing are provided by Toyota Racing Development. TRD thought they had resolved the issues experienced in California, but according to Lee White of Toyota, they actually went in the wrong direction.

“We’re going to use a heavier lubrication and not try to squeeze every ounce of horsepower out of them,” White said. He estimated the cost of using the new lubricants would be four to five horsepower.

The issue appears to be wear between the camshaft and the lifter. White said “It’s either a coating, lubrication, lack of lubrication, too much lubrication, not enough coating or a material situation or just the simple fact that we haven’t been testing.”

NASCAR engines use Flat tappet camshafts. The camshaft is a rotating shaft with lobes. The flat tappet lifters ride along the lobes and move the pushrods. The pushrod is attached to the rocker arm, the rocker arm is attached to the valve, so as the cam shaft rotates, the lobe profile determines how rapidly the valves open and shut.

Camshafts for racing usually have ‘radical’ lobe profiles. The cams on your car let the valves open and close at something of a leisurely pace. An engine going 9500 rpm opens and closes each valve 79 times each second. To get the most air/fuel mixture into the car and the most exhaust gas out as fast as possible, the valves must open and close quickly. The cam lobe profile is designed to achieve this.

As you might imagine, there is a lot of rubbing between the cam and the lifters. Roller lifters have a bearing that rotates, but flat lifters pretty much just rub. NASCAR mandates flat tappet lifters and that means a lot of friction. One way to decrease friction is with lubricants like oils. The oil gets between the two moving pieces and lubricates the interface.

But there are issues with oil. When the pressure between the moving parts gets high, it squishes the oil from between the parts. You have to use thicker oil that can stand up to the pressure; however, thicker oil increases friction in its own way because the parts have to move against the oil.

There are two issues: friction and wear, both of which can be addressed by coatings. Very thin (a tenth of a human hair’s width) coatings are deposited on the cams and/or lifters. Wear is two materials abrading each other. When the parts wear, they get smaller, they don’t contact they way they are supposed to, and they often get rougher. Friction, the resistance to motion of two things sliding past each other, costs horsepower in the engine.

Nitrides (chromium nitride and titanium nitride) are very hard ceramics that minimize wear. (Titanium nitride is the gold-colored coating you see on drill bits.) Coating a part is done by putting it in a vacuum chamber, removing most of the air molecules, then vaporizing your metal (titanium or chromium) and introducing some nitrogen. These types of coatings are often used on the parts of valves that contact the valve seats.

Friction is a second problem because engine power has to be used to overcome friction and that takes away from the power that goes to the wheels. Carbon comes in different forms. Two are graphite, which is one of the softest forms, and diamond, which is the hardest mineral known. Both are crystalline. If you make carbon amorphous-meaning that the atoms don’t have a regular arrangement-it’s called ‘diamond-like carbon’ or DLC. DLC has the best of both world-hardness and low friction. DLC has 10x less wear than titanium nitride and a lower coefficient of friction (which means its more slippery). DLC is actually a little too hard in some cases: it is fairly brittle, so it can crack if it gets hit too hard. Metal-doped DLC is not as hard, but also not as brittle. Since NASCAR engines often use ‘lofting’ (the lifter actually loses contact with the cam lobe), there’s a lot of impact when the pieces come back together.

Back at Daytona in 2007, a number of teams had issues with coatings that came off cams and/or lifters. I’ve been working on an article for the Materials Science Research Bulletin on materials used in NASCAR and talked to a number of people in the coating industry. They all stressed the following: Failure of coatings can happen in at least two ways: One is a processing failure of some type - a mistake was made by the company doing the coating. The second is when teams push the coatings past what they can take. Most coatings are two to four microns thick. A human hair is about 70 microns thick. It’s akin to teams running extreme camber on the front tires and then blaming Goodyear when they blow tires. (I should mention that none of the people I talked to supply cam or lifter coatings to TRD.) While coatings extend the properties of the parts, they don’t work magic. We haven’t yet invented a coating that requires absolutely no oil.

Both the engine people I spoke with said that, if they had tested at Las Vegas and California, the increased rpms would have been apparent and they would have stepped back a bit. It would be an interesting calculation to see how much the teams saved on testing compared to the costs of all those blown engines.

For more information

• Circle Track - The Science of Cams
• Circle Track - Flat Tappets and Lubrication