Eliminating Restrictor Plates?

 Daytona, Engines, rear-end gear, Restrictor Plates, Talladega, transmission  7 Responses »
Oct 052012
 

Every return to a restrictor plate track brings suggestions about how we might eliminate the restrictor plate.  Restrictor plates serve the very necessary function of limiting car speeds at Daytona and Talladega so that the cars stay on the ground.  The negative is that they remove throttle response.   One suggestion from some readers that I hadn’t heard of before suggested that NASCAR could just change the rear-end gearing parameters to shift the power curve and reduce horsepower that way. Will that work?

The amount of horsepower an engine make depends on the rotation rate of the engine.  The faster the engine runs, the more combustion events and the more power generated.  This works up to a point, because if you rotate the engine really fast, you start having problems getting enough air into the engine and the power goes down.  The graph below is for a typical unrestricted engine that makes its maximum horsepower around 9300 rpm.

In order to cut the horsepower back to what you’d need to run safely on a plate track, you would need to restrict the engine to run at about 450 hp – which would mean the engine would have to rotate at about 4500 rpm.

Looking at the curve above, it’s evident that the engine is designed to run at its peak horsepower.  What dictates that curve?  Cylinder displacement, engine configuration, head configuration, etc.  But mostly  NASCAR determines the curve because of the rear end gear rules.

NASCAR engines run up to about 10,000 rpm (revolutions per minute).  Rpm is a unit that measures how fast something rotates. It’s like miles per hour, but miles per hour corresponds to a linear motion rather than a rotational motion. The minute hand on your clock, for example, makes one revolution every hour. The seconds hand makes one revolution per minute.

The circumference of a typical tires is around 88 inches.   Every time the tire rotates once, the car moves 88 inches, so 1 tire rpm = 88 inches per minute.  You can convert this into miles per hour.  Since I chose a nice round number like 88  inches for the tire circumference, it works out to a really simple equivalence:  1 tire rpm = 1/12th of a mile per hour.  This means that if you want to go 200 mph, the tires have to rotate at 2400 revolutions per minute.

The power curve above shows that the engine makes the most horsepower at 9300 rpm.  This produces a problem:  the engine is driving the car at 9300 rpm and the wheels are rotating at 2400 rpm.  That’s why you have a transmission and a rear-end gear, as illustrated at right.  The diagram shows the gear ratios for a Borg Warner MM6 manual transmission and a GU6 3.42 rear-end gear, as might be found in a Corvette.  Note that NASCAR cars are not allowed to use any gear that increase the rotation rate between the engine and the wheels.  No 5th or 6th gear, either.  1:1 is the best you are allowed — which means that the rotation rate coming out of the transmission is the same as the engine rotation rate.

At maximum speed, the transmission is using a 1:1 gear, so the only reduction occurs at the rear end gear.  A 4:1 gear means that one gear makes four rotations for every one rotation the other gear makes.  If the engine is rotating at 10,000 rpm, and it passes through a 4:1 rear-end gear, you have 2500 rpm at the tires (which is 208 mph).

The whole point of this discussion is to keep the cars at lower speeds so they stay on the ground.  Let’s say you want to limit the cars to 190 mph – that requires the wheel rotate at 2280 rpm.  We don’t want more than 450 hp, so the rear-end gear has to take the rotation rate from 4500 rpm to 2280 rpm, which means 4500/2280 = 1.97, so you need essentially a 2:1 rear-end gear.  (Just for comparison, a typical rear-end gear is 3.3-3.9, depending on the track.)

So it is possible to gear the car down so that it simply doesn’t produce as much horsepower.  It is a better solution than what we currently do?

Restrictor plates work by reducing the air coming into the engine, which means you can give the engine less gas and thus you produce less horsepower.  Gearing down would reducing the horsepower by making the engine run in a much-less efficient range.

One consequence of a lower rpm is that you would have really back problems with knocking.  Knocking happens when the air-fuel mixture auto-ignites (ignites before the spark plug fires).  Knocking is much more likely at low engine speeds because the the combustion happens so much more slowly than in a fast-running engine.

Another consequence is that my engine design friends tell me that they can probably tweak an engine, within the rules, to produce more horsepower at 4500 rpm such that NASCAR would have to further change the gear and run the engines at 3500-4000 rpm, which exacerbates the knocking problem.

I wondered whether taking the engine speed down might increase throttle response, but none of the experts I spoke with thought that it would.  The problem, they say, is that as long as cars are running at full out power, aerodynamics will dominate plate tracks.  You’d have to decrease downforce and increase drag to really make a difference.

Finally, there’s an aesthetics issue.  The sound of an engine changes with its frequency.  If you went to Indy and we blindfolded you and asked you tell us whether the car on the track was a NASCAR racecar or an Indy car, it would be easy:  Indy (and F1) cars sound like mosquitoes.  They run at about twice the speed of a NASCAR engine.  If you forced a NASCAR engine to race at 3500-4500 rpm, the car would sound like it was in pain – you’d get a low moan instead of the engine sound associated with 9000 rpm that we’ve all come to know and love.

If you were paying attention, you ought to be wondering why you couldn’t just run at very HIGH rpm – the power curve goes down on each side of the peak, so you could have the engine run at 13,000-14,000 rpm and output 450 horsepower.  The problem on that side is that NASCAR initiated the gear rule so that teams wouldn’t have to deal with the incredible stress on engine parts that have to run at those very high speeds.  High-speed engines would significantly increase the cost to teams – it would be cheaper in the end to just let them build a dedicated open (not restricted) plate track engine.

In conclusion, yes – gearing down would work in theory, but it would introduce its own unique problems that would offset the advantage.

Why was Michigan so hard on Motors?

 Chad Knaus, Engines, Hendrick Motorsports, Jeff Gordon, Jimmie Johnson, Michigan International Speedway, Tony Stewart  3 Responses »
Aug 232012
 

The Hendrick engine shop had four failures at Michigan.  The 24 and the 14 reportedly both had valve spring failures.  The worst was the 48, whose engine went south while leading with only six laps remaining.  Jimmie Johnson drove the car up to the hauler and walked back to his motorcoach with his helmet on, not talking to reporters.

I don’t blame him, especially when you realize how close he got before the motor let go.

High, Sustained RPM

Michigan is one of the tracks where the speed at which the motor rotates stays constant throughout an entire lap.  Watching the numbers from the television, most motors changed from only 7800 to 8500 rpm (revolutions per minute) throughout a lap.

Engine Diagram

Number of laps, or even miles are not the best way to gauge engine use because there is a huge difference between running at 8000 rpm and running at 3000 rpm.  What’s important is how many times a part is called upon to do it’s job.

The valves (one intake and one exhaust) are raised and lowered by the rotations of the camshaft (as shown above).  The camshaft is driven by the crankshaft.  When we say an engine is running at 9000 rpm, we mean that the crankshaft makes nine thousand rotations every minute – or 150 rotations every second.

Here’s the critical part:  The camshaft makes one rotation for every two rotations of the crankshaft in a four-stroke engine.  At 9000 rpm, the camshaft is running at 4500 rpm, which translates to 75 openings and closing of the intake (or exhaust) valve every second.  This means that the valve spring compresses and expands 75 times each second.

This is a linear phenomenon.  If the engine runs half as fast, each of these things happens half as many (37.5) times each second.  The faster the motor runs, the more movement, the more rubbing of parts and the more opportunity for pieces to break.

Watch the numbers this week at Bristol – you’ll see a much larger difference in speeds as the drivers slow down through the corners and accelerate through the straightaways.  Even more importantly, watch the changes in engine speed coming up next week at Atlanta, where you’re going to see similar high, sustained speeds.  The same issues will be in play for Charlotte and Texas.  This may just have been a case of a box of sub-optimal valve springs, or the engine shop may have been trying a more aggressive setup in preparation for similar track in the Chase.  I’m not worried – they’ll get it figured out (if they haven’t already).

By the Numbers

Let’s do a quick calculation.  The race time was 2 hours, 46 minutes and 44 seconds to run 201 laps.  There were 35 laps of caution, so (35/201=)17.4% of the race was run under caution and 82.6% of the race was run under green.

2 hours, 46 minutes and 44 seconds is 10,004 seconds.  82.6% of that is 8,263 seconds that were run under green.  If we take an average of 8000 rpm, which is 66.6 revolutions of the camshaft every second, the average valve and valve spring went through half a million up-and-down cycles.

Jimmie Johnson ran a top happy hour lap of 36.323 seconds.   Assuming an average of 8000 rpm, each lap at that speed adds another 2,421 cycles of the valve spring. Six laps means he was short 14,526 out of over a half-million cycles.  Think about sixteen valves and valve springs that make well over a million (including practices) successful executions and come up short by a few tens of thousands.

No wonder Johnson didn’t want to talk to the press.

 

Plate Racing Rules: Getting Ready for Talladega

 Cooling, Engines, radiators, Talladega  No Responses »
May 032012
 

Most of the issues we were talking about at the start of the year regarding the measures NASCAR has taken to eliminate or reduce the two-car draft are still in play, so I thought I’d put the most important in one place as you start getting ready for Talladega this weekend.

One of the major changes is the radiator: The water capacity was decreased, which means that it can’t cool as effectively as it could with a larger volume of water. That limits how long cars can draft together in close formation, where the trailing car’s radiator is blocked and doesn’t get as much air circulating.

A related issue is the small, but extremely important limiter on the radiator called a pop-off valve This is one of the easiest last-minute changes NASCAR can make to adapt to changing temperatures — and new innovations teams have made to get around the current rules.

Finally, it seems as though bump drafting has gotten harder to do correctly. It’s all a matter of preventing cars from getting torqued. Literally.

Kansas Wrap Up: What Caused all the Engine Failures?

 Andy Randolph, Engines, Kansas, rear-end gear, transmission  5 Responses »
Apr 262012
 

The defining characteristic of the Kansas race was the surprising number of engine problems.  Many of those problems can be attributed to the change in rear gear from a 3.89 to a 4.00.  At  190 mph at a track like Kansas, your wheels make 2270 revolutions per minute (rpm).  If you watch the telemetry on the television broadcast, you know that the engine is rotating around 9500-9900 rpm.  Since the engine is attached to the wheels, there has to be something to change the rotation rate between the engine and the gears.

Gearing Up

That something is the transmission and the rear gear.  As shown at right (with the values given for a Corvette ZR-1), the engine rotation passes through the transmission and then through the rear-end gear before reaching the wheels.  A 4.00 gear means that the ratio of rpm in to rpm out is 4.00:1.  It takes four revolutions of the input to produce one revolution on the output.  If you have something rotating at 8000 rpm and you add a 4.00 gear, then the rotation is reduced to 8,000 rpm/4.00 = 2,000 rpm.

Note that NASCAR does not allow 5th or 6th gears and does not allow overdrive (when the first number is smaller than the second).  The lowest gear you can have is 1:1 in NASCAR.

Let’s compare running at 190 mph with the two different gears.  Last year, a 3.89 gear was used. At 180 mph, you’d better be running in 4th gear (which means 1:1 and the speed coming into the rear end gear is the same as that coming from the engine.  The engine speed required to go 190 mph is this 3.89*2270 rpm = 8830 rpm.  This year, with a 4.00 gear, you’d need to be running at 9080.  If you’re running 200 mph, last year you needed 9293 rpm and this year it would be up around 9556 rpm.  You’re basically running 250 rpm (or so) higher this year than last year at the same speed.

Andy Randolph, Engine Technical Director at ECR Engines tells me that engines were running at 9800 rpm for sustained times.  Although the engine rotates that fast at some places, doing it continuously places huge stress on the mechanical parts – that’s why most of the failures were due to mechanical breakage.  (Because I know he’s too modest to mention it, I’ll point out that none of the engines that had problems at Kansas were from ECR.)

The Math

For those of you wondering about where my numbers come from, here’s a calculation I did for Las Vegas.  The only difference is the slight variation in tire circumference.  If you plug in the numbers to the formulas and don’t get what I got above, I probably screwed up on the calculator.

Left-side and right-side tires have difference circumferences.  The circumference of a left-side Vegas tire in 2008 was 87.4″, while the right-side tires had a circumference of 88.7″.

To calculate how many times the tires rotate each minute, I first convert the speed into inches per minutes.  I know to use those units because I’m trying to get an answer in revolutions per minute, so I need to convert hours to minutes. I also know that every time a right-side tire makes one complete rotation, it has traveled 88.7 inches, so I’m going to convert miles to inches because I know I will need that later. Convert 45 mph to inches:

45 mph corresponds to 47,520 inches per minute. Looking at the right-side tires (for no particular reason), the car travels 88.7 inches every time it makes one full rotation. The number of times the tires rotate each minute is 536 rpm, as shown below.

 

 

 

Popping Off: Breaking the Two-Car Draft by Heating up the Engines

 Cooling, Daytona, Engines  2 Responses »
Feb 162011
 

Any closed vessel that is subjected to high temperature will experience increasing pressure.  When that pressure gets high enough, we change from calling it a pressure vessel to a bomb because if(when) it explodes, the vessel itself becomes a collection of high-speed projectiles.  For safety, we don’t heat closed containers if there’s a chance they will reach high enough pressure for them to explode.  A pressure cooker, for example, has a relief valve that at one time was as simple as a rubber stopper tightly fitted into the lid.  The rubber stopper fit in the hole securely enough to handle up to some cutoff pressure, then popped out when that pressure was exceeded.  (This is not an ideal safety mechanism because the flying stopper can injure someone, as can the blast of steam that dislodged the stopper.)

A more practical version is a valve that automatically opens when the pressure exceeds some cut-off value.  The open valve allows excess steam (and sometimes water) to escape.  As soon as the pressure is below the cut off, the valve closes again.  In addition to being safer, it eliminate the time-wasting step of looking for the stopper.

The cooling system on a car is a prime example of a closed system that is heated to high temperature.  Water is pumped through holes in the engine block, where it collects heat.  The now-hot water moves out of the engine and into the radiator, where the heat is transferred from the water to air surrounding the radiator.  The cooled water returns to the engine to pick up more heat.  A Sisyphusian task, indeed.

A radiator is a twist of metal tubing onto which is fastened thousands and thousands of fins that help cool the water that circulates through it.   A typical stock car radiator (like the one at left) might have 20 fins per inch (compared to 10 fins per inch on a typical car radiator).  The more fins per inch, the more surface area available for exchanging heat between the radiator water and the outside air; however, air has to pass through the radiator, so if there are too many fpi, the air flow is decreased and that lessens the cooling.

The water can only carry away so much heat on each trip, so the water temperature gets hotter and hotter as long as the engine keeps producing heat.  The water increases in pressure as the temperature increases.  (See Equation, Clapyron for more on that.)   Water, of course, boils at 212 degrees Fahrenheit, and that would seem to set a limit on how hot you can run an engine; however, there’s a caveat.  Water boils at 212 F only at atmospheric pressure.  As the graph below shows, the higher the pressure, the hotter the water can get before it boils.  Atmospheric pressure is right about 14.7 psi, and that’s where the 212 degree Fahrenheit number applies.  But if you can get the pressure of the system up to about 45-48 psi, the water won’t boil until 275-280 F.  If you can maintain a high pressure in your radiator, you can prevent the water from turning into steam.  Water is much better at carrying away heat than steam is.  Water also flows much better.  Most radiators have a pop-off valve that blows when the pressure gets too high.  A typical radiator cap on a car would be about 15 psi, which actually means 15 psi above atmosphere.  Atmosphere is 14.7 psi, so you’re looking at about 29.7 psi in absolute terms.  This is why your radiator cap has all of those warnings about not removing it while the car is hot:  when the system is vented (opened to atmosphere), the super-hot water will turn into super-hot steam and gush from the opening.

A pop-off valve serves as a ‘weak link’: it has to blow before anything else in the system blows.  Most radiator caps on passenger cars are spring loaded:  When the pressure gets too high, the cap lifts off its seat, opening the system and allowing the hot water to escape into a reservoir.  As soon as the pressure is back down, the radiator cap goes back to being closed.

In a NASCAR car, the pop-off valves open and route the escaping steam and/or water through a tube that passes up near the right-hand side of the car’s windshield.  When you see a car “pushing water”, the maximum pressure has been exceeded and the pop-off valve opened.

For the last couple of years, most of the top NASCAR engine shops have focused on strengthening radiators.  It’s not difficult to get a pop-off valve set to 100 psi.  The problem is that if the pop-off valve isn’t the weak leak in the system, something else breaks.  It’s much more expensive to replace a radiator than a valve – so the size of the pop-off valve is really limited by the strength of the radiator.   A stronger radiator allows a higher pressure to be maintained.  Tim Brewer said that teams were pressurizing their systems to 80 psi (which would be 94.7 psi on my graph were it to extend to the right.)

Two-car drafting produces very high speeds, and that makes NASCAR nervous.  Cutting down the restrictor plate (which they did today) slows down the cars, but NASCAR doesn’t want to change the plate more that 1/64th of an inch or two because the change in plate size significantly affects how air enters the engine.  Teams have been designing engines around a particular plate size, although you would think that by now, they’d know to test not only the announced size, but plates one or two sizes up and down.

The limiting factor on how long two cars can stay in a draft is temperature.  The air intake of the trailing car is blocked when it is drafting, and the water temperature increases.  Two cars could go twenty laps or more before they had to separate.  NASCAR’s plan to limit the two-car draft started with a mandated pop-off valve.  NASCAR requires all teams to use a 33-psi pop-off valve, which corresponds to (33+14.7=47.7 psi) in my graph above.  All the work teams did to manage an 80 psi pressurized system is now out the window.  They also decreased the size of the opening through which air enters the car to cool the engine.  Less air reaching the radiator means less heat transferred from the water and a warmer engine.

Now if someone only could come up with a pop-off valve for drivers…

*****

EXTRA:  Wondering about the different between a tapered spacer and a restrictor plate?  Check out this video, which illustrates very visually how a fluid flows differently through an orifice (the plate) and a nozzle (the spacer).  They’re using both on the Nationwide cars now.  The way the air enters the engine really makes a difference in the combustion dynamics.  Making a smaller spacer would have created too big a perturbation.  The holes in the new space are actually larger, but the plate will help decrease the overall flow.