Engine Issues at Talladega: Vapor Lock, Gas Cans and Oil Coolers

 Cooling, Electronic Fuel Injection, Engines, NASCAR, Oil, radiators, Talladega  7 Responses »
May 072012
 

An usual number of teams “ran out of gas” or had engine troubles during the Talladega race.   The TV analysts had some ready answers for what might have caused these problems.  Their extemporaneous theories tend to elicit sighs from engine builders, who know that problems can rarely be diagnosed at the track – and even more rarely by someone who hasn’t looked at the car.

A wonderful aspect of blogging is that we’re not called to have answers on the spot like the television broadcasters and we have the leisure of time.  Let’s examine some of those theories.

The Gas Can

SPEED reported engine builders suggested that the teams weren’t getting a full tank of fuel into the car and that’s why they were running short.  This doesn’t really make sense given how fuel mileage is calculated.

Prior to putting the gas in the car, the gas can and gas are weighed.  After the fueling is complete, the gas can and any remaining gas are weighed.  The difference between those two weights is the weight of gas that actually got into the car.  This assumes that all of the gas missing from the can made it into the car.  If gas is spilled, it will affect the validity of the calculations.

Sunoco provides the teams with the density of the gasoline – how much one gallon of gas weighs.

You can calculate the volume of gas using this information.  For example, if the density of gas is 6.073 lbs/gallon and you find that you’ve put in 133 lbs worth of gas, the volume of gas that got into the tank.

(Yes, I know I’m using weight instead of mass, but as long as both the density is given in terms of weight/volume, the ‘g’s cancel and we’re OK.)

Teams know from this how much fuel they actually put into the car and they base their calculations (and what they tell the driver) on these numbers.  So even if they aren’t getting a ‘full tank’, the crew chief is well aware of it.  It doesn’t matter whether the lack of fuel is due to human error or a malfunction on the part of the gas can.  This is not a likely cause of the fuel problems we saw Sunday.

There are two places that this might be an issue:  First – if the engineer does the fuel mileage calculations incorrectly, you’re going to have the wrong prediction for the number of laps you can run.  The problem with this is that it is highly unlikely that multiple groups from different teams made the same mistake in the same direction.  The number and breadth of problems suggests something more systemic.

The second issue is that the density of fuel depends on temperature.  Fuel becomes less dense at higher temperatures, so putting in the same weight volume would mean less volume fewer molecules. (Thank you Barry!) It’s a little confusing because all of the calculations the team makes are done in terms of gallons, but that assumes a particular density. I’m checking to see whether teams take this into consideration.

Vapor Lock

Another theory proposed on the network broadcast (as a result of a crew chief comment, I believe) was “vapor lock”.

What is Vapor Lock?

Vapor lock happens when liquid fuel vaporizes (changes to a gas) prior to entering the combustion chamber.  The pumps in a fuel delivery system are designed to pump liquids, not gases.  The fuel pump cannot pump gas well, so the fuel pressure drops and fuel stops being delivered to the engine.  Since engines don’t run without fuel, the car ‘locks’.

How easily a fuel causes vapor lock depends on its vapor pressure:  the higher the vapor pressure, the more susceptible the fuel is to vapor lock.  (Although it’s not relevant to Talladega (elevation 596 feet), vapor pressure increases at high altitudes and this may also cause vapor lock at high altitudes, even when the car behaves fine at lower altitudes.)

Does EFI and/or Ethanol Cause Vapor Lock?

Vapor lock is LESS likely to happen with EFI than with carburetors.  The NASCAR carbureted system ran at low pressure and lacked a fuel pump in the fuel cell.  Those factors made it much easier for the engine to vapor lock.  The EFI system runs somewhere around 70 psi and has a fuel pump inside the fuel cell, which decreases the probability of vapor lock.

Vapor lock can happen within the engine (prior to the cylinder) or at the fuel pick-ups in the fuel cell.   The most likely place for vapor lock to be initiated would be at the fuel pick-ups because the fuel cell itself isn’t pressurized; however, the two engine builders I spoke to this morning both said that none of the data they have indicates that vapor lock was an issue in their cars.

Ethanol also makes it LESS likely that a car would experience vapor lock because ethanol has a lower vapor pressure than gasoline.  Ethanol-containing fuels are less likely to vapor lock than pure gasoline.

So What IS the Issue?

My sources suggest that high oil temperatures are causing the engine problems.  This problem is exacerbated by high outside temperatures and the reduced cooling inherent in the rules package that was implemented to prevent the two-car draft.

Two fluids help cool the engine:  water and oil.  Both are in turn cooled by the air coming in through the grille.  As the air flows in through the grille, it first encounters the radiator used for cooling the water circulating through the engine.  The air comes in at temperature Temp 1 and leaves at temperature Temp 2, where Temp2 is larger than Temp 1.  The air picks up some of the heat from the radiator and carries it away, which is why Temp 2 is larger than Temp 1.  (For more on this, see my blog on radiators)

Behind the water radiator is another cooler for the oil.  It also depends on cool air coming in through the grill.  The problem teams are having is that Temp 2 is so high that the air can’t cool the oil efficiently.  The problem is exacerbated because 1) the cooling air coming in (Temp 1) is hotter due to the outside temperature and 2) the air is warmer after passing through the water cooler because the engines are running hotter.

Thanks to the EFI data, teams can look at how the temperatures change in a much more detailed way than they could back when they relied on the driver relaying temperatures.  My engine guys report that they are seeing a difference of up to 50 °F between Temp 1 and Temp 2.  That difference is normally only 15-25 °F.  In addition, Temp 1 is higher to start with when the external temperature is high like it was at Talladega on Sunday.  (And like it no doubt will be in Daytona in July.)

Oil is a combination of different types of long-chain hydrocarbon molecules that unfortunately break down at high temperatures.  If you’ve ever heated oil on the stove above its smoke point, you’ve seen firsthand the decomposition of oil molecules due to high temperature.  The result is usually a gummy dark tar-like substance deposited on the pan surface.

The same thing happens with engine oil:  when it starts to decompose, it can’t lubricate the engine. An engine cannot run at peak power for very long without functional oil.

Yes, I did suggest that it would make sense to put the oil and water coolers in parallel instead of in series so that some of the cooler air would get to the oil cooler without having to pass through the water cooler first.  No dice – it’s been tried and deemed to be against NASCAR rules.

 

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.

Why Does Bump Drafting Seem Much Harder in 2012?

 Aerodynamics, Cooling, Daytona  No Responses »
Feb 252012
 

Although there is a lot of science behind bump drafting, the act of bump drafting is an art.  Even the experienced bump drafters are surprised by the touchiness of the cars this year.

What’s a Pop-Off Valve — and Why You Need to Know for the Daytona 500

 Cooling, Daytona, Engines, NASCAR  3 Responses »
Feb 212012
 

Note added 14:14 2/22/12:  OK, I did predict that this was likely to change.  You can look at the chart on the video and see that the temperature at 28 psi is about 271 F.

One thing you will hear a lot as soon as coverage of practice starts Wednesday will be speculation about possible changes to the pop-off valve on the radiator.  What is a pop-off valve and how will it affect the Daytona 500?  Here’s the answer:

One of the reasons for the focus on the pop-off valve is that it’s one of the most easily adjustable pieces of the strategy to prevent tandem drafting.  If the temperatures are warmer, NASCAR can raise the pop-off valve pressure if they think there might be a need for more cooling.  EFI makes changing restrictor plate sizes a bigger deal than it used to be – so if there are changes, this (and the size of the grill opening) are the most likely places for them to happen.

The Motorsports Science Minute Video: Radiators at Daytona

 Cooling, Daytona, Engines  4 Responses »
Jan 102012
 

Thursday marks the first time we’ve had an open test at Daytona in a couple of years.  With the myriad rules changes aimed at getting away from two-car drafting, the teams are going to need to make the most of these sessions — especially if NASCAR opts to make more changes before Daytona
 

Below, a short video explaining why radiators are such a big deal at Daytona this year.  As always, happy to answer questions you might have! Drop them in the comments and I’ll reply. Or send them to me @drdiandra on twitter.

turning left, shifting right: why drivers move to the right to get air to the engine

 Aerodynamics, Cooling, Daytona, Engines  5 Responses »
Feb 222011
 

Jack asks:

I’m curious as to why the rear cars are offsetting to the right, when offsetting to the left would let the rear driver see what is happening ahead of them and keep the radiator in cooler air, since the exhaust on these cars is on the right. I know that all those drivers and crew chiefs are smarter than I am, so I must be missing something.

Thanks for the question, Jack. Give yourself a little more credit: you bring up some really good points that I bet a lot of people didn’t see.

Drafting at Daytona has become more important than ever, with the two-car draft being the most effective means of getting speed. The problem is that this mode of drafting completely blocks the front grille, and that limits how much air gets in to cool the engine.  The trailing car has to back off to let air into the grill when the engine gets warm.

Jack noticed that everyone was shifting to the right.  I think it’s a matter of simple geometry and the fact that NASCAR is chiral.  Chiral means simply that something twists one way.  All of your DNA twists in one direction.  NASCAR drivers turn (with two exceptions a year) right left.  (Note:  Thanks to the commenter.  What WAS I thinking there?)

Below, I’ve drawn two cars trailing each other in line on the left, the trailing car shifted to the right (middle) and the trailing car shifted to the left (right).

When cars turn left, a natural gap opens up on the right-hand side between the cars.  Moving to the right takes advantage of the gap and makes it slightly larger.  If the trailing car moves to the left, I don’t think it’s going to get as much air.  So despite the possibility of being able to see better, going to the left doesn’t look as effective to me as shifting to the right is if the goal is to get the most air into the engine.

Thanks for asking the question, Jack!  I always read the comments, so if you have a question you’d like answered, please leave it in the comments for me.

 

 

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.