Can’t stand the heat? Get out of the engine.

I ran across an article not long ago in the Technology Review about improving the efficiency of the internal combustion engine. The technology it describes turns out to be nothing more than a particularly clever combination of several technologies that have been around for longer than I have been alive, but which when properly used in conjunction generate much larger increases in performance and efficiency than anyone has previously been able to extract.

I find such stories of people combining old dog tricks to obtain racing greyhound results very interesting reading. In addition, the very fact that people are still squeezing such remarkable gains in efficiency out of something as venerable as the internal combustion engine suggests to me that predictions of its imminent demise in the face of rising energy prices are likely to prove premature. It may well be with us for quite some time yet.

There is a strong tendency for established players to want to stick with those things that have already made them successful, and automotive manufacturers, especially in the United States, are nothing if not established. They got to be as big and established as they are on the strength of the internal combustion engine. It is a technology with which they are very comfortable. This means that if they have the option, they’re likely to stick with it for as long as possible, rather than risk venturing out into new and untested waters with some new system for powering cars.

The technique the researchers in the article have perfected is mostly one of preventing an engine from knocking. The vast majority of internal combustion engines in use today are reciprocating piston engines. In order to generate power, they have a piston which resides in a cylinder, moving up and down. Modern engines are four-stroke engines, which means that the piston moves up and down twice (four total movements, two up and two down) for each full cycle.

To begin with, the piston is all the way at the top of the cylinder, leaving a very small gap above the head of the piston. A valve on one side (or, in many newer engines, a set of two valves) opens up as the piston begins to move down the length of the cylinder. As the piston moves down, it sucks air into the cylinder through the valves. This air has already been mixed with gasoline in the pipe the valve opens into by the injector nozzles, which break the gasoline into very tiny droplets, so that it can mix more thoroughly with the air, and then spray it into the air before it enters the cylinder. This is the “intake stroke”.

Once the piston reaches the very bottom of the cylinder, the valves close, sealing the mixture of fuel and air inside. The piston then begins to move back upwards, compressing the mixture as the space above the top of the piston shrinks. The faster this happens, the more rapidly the mixture compresses, and the more it heats up under the pressure. This is the “compression stroke”.

As the piston reaches the very top of the cylinder, the fuel air mixture reaches its highest compression. Just as the cylinder passes the top and begins back down again, the spark plugs, located at the very top of the cylinder, fire, igniting the mixture. As it explodes, the fuel burns, consuming the air, and the entire thing expands as it heats up and burns. This drives the piston back down to the bottom of the cylinder. It is this process of the burning fuel and air pushing down on the piston that drives the engine through the rest of the cycle. This is the “power stroke”.

Once the piston reaches the bottom of the cylinder, if it was mixed in the right ratio, the fuel and air mixture should be just reaching the point of being entirely burned out. At this point another set of valves at the top of the cylinder open, and the piston begins to move back upwards again. In the process, it drives the burnt fuel residue out of these valves into the exhaust system of the car. This is the “exhaust stroke”. When the piston reaches the top of the cylinder, the valves close again, the the entire cycle is ready to begin anew.

The difficulty with this is the competing demands of extracting ever greater performance from an engine versus the immutable laws of physics. It turns out that no matter how much performance you might want, physics always wins the argument about how much performance you can actually get. The basic principals of the piston engine haven’t changed for hundreds of years, since the days that they were powered with steam instead of gasoline. Virtually all of the advances in design since that time have been directed towards making the system leaner and more efficient, thus able to generate more power from a smaller, less expensive package.

In particular, the more you can squeeze the fuel and air mixture during the compression stroke, the greater the power of the engine. In general, this is accomplished by having the piston travel up and down farther on each stroke. This means that a larger total volume of the cylinder is compressed into the same tiny space above the top of the piston at the end of the stroke. Because the mixture is then at very, very high pressure, when it ignites, it can push the piston back down farther, and with greater force. The result is more power (in terms of both horsepower and torque) for the same amount of gasoline burned.

The problem is that as the fuel and air compress, they heat up. The faster they are compressed, the greater the heat. If the heat rises too quickly, the mixture will ignite before the spark plug fires, and in fact is likely to ignite before the piston reaches the top of the cylinder. This is a huge problem, because it means that as the piston is trying to continue upwards to complete the compression stroke, the fuel is already burning, trying to expand and pushing back down on the piston.

In the end, the piston usually wins the contest, because the momentum of the drive shaft that it is connected to, and through it, the entire drive-train of the car, is great enough to compress the mixture anyway. The problem is that by the time the piston is ready to go back down again, some or all of the gasoline has burned, and it isn’t able to extract any power from the burning fuel, which is the entire point of the exercise in the first place. In the meantime, it has used up energy from the other pistons in the engine, which could otherwise have been used to drive the car forward. The pistons in an engine all fire in sequence, meaning that if the engine has at least four pistons (and almost any car has at least that many), one of them is in the process of undergoing a power stroke while the piston we’re looking at is performing its compression stroke. This robs the engine of power, further reducing the efficiency of the engine. While it’s at it, it also tends to make an unpleasant sound as the moving parts of the engine try to go in different directions. This sound is called a “knock”, and it is from this sound that the phenomenon gets its name, “knocking”.

As a side note, this is why high performance cars require premium gasoline. Higher performance engines have higher compression ratios, which is to say, the fuel and air mixture gets squeezed more during the compression stroke, than lower-performing cars. High octane gasoline contains a higher percentage of the hydrocarbon molecule octane in it. Octane is eight carbon atoms in a chain. By contrast, lower grade gasoline contains more heptane, which has only seven carbon atoms in a chain. Octane is less likely to combust under pressure than heptane is, which means that higher octane gasoline can withstand higher compression ratios without knocking. However, this is also the reason you should only ever use the lowest possible octane rating gasoline in your car that meets the minimum specified for the engine, in a sort of reverse Price is Right scheme. This is because heptane actually burns better, gallon for gallon, than octane (to be specific, it burns more quickly).[1][2]

In order to be able to produce engines with higher compression ratios, and therefore better power for the same quantity of gasoline, it is necessary to address this issue of premature combustion. One way to do this is to require that your engine only use a higher grade of gasoline. Another option, which does not require your customers to fill their tank with premium, is to find a way to decrease the temperature inside of the cylinder. Over the years, clever engineers have found a variety of ways to do this.

Perhaps the most obvious is to cool the engine block. This is why modern cars have radiators and cooling fans and any of a wide variety of other systems designed to pass either water, oil, or some other coolant through the metal around the cylinders, drawing away some of the heat. This has been effective enough to get engines to the point they are at today, but not enough to push them further.

Another technique, known for some time, is to add something into the cylinder itself. Various approaches have been tried, but all rely on pushing something cold into the area above the piston, lowering the temperature inside. The technique discussed in the article linked above falls into this category. In particular, they are injecting ethanol.

Ethanol is desirable in this regard for two main reasons. First, it is flammable, and in fact is already added to gasoline in the United States. This alleviates the problems associated with squirting water or other non-flammable liquids into a space that needs to be host to an explosion in the very near future. Secondly, when it evaporates, ethanol gets very cold. Thus, if high-pressure, liquid ethanol is injected into the cylinder, it rapidly expands and evaporates, and it cools the area around it as it does.

Rather than adding the ethanol to the mix at the same point that the fuel and air are combined, in the pipe leading up to the inlet valves on the cylinder, this technique injects it directly into the cylinder. This increases the effect of the cooling, which occurs directly against the metal parts that were recently heated by the explosion that occurred during the previous power stroke a fraction of a second before. This, too, is something that people have known about for decades, using ethanol, propane, and any of a wide variety of other agents, but in this case the researchers seem to have perfected the technique to an extent earlier pioneers didn’t achieve.

To this technology, the researchers have also added another golden oldie. Namely, a turbocharger. This is, in simple terms, a high-pressure fan. It is powered by a turbine in the exhaust system that spins as the exhaust gas pushes past it. This, in turn, drives a compressor fan on the air intake system for the engine that compresses the air being fed into the cylinders. This allows the cylinders to take in more air in a shorter period of time, allowing them to burn larger quantities of gasoline with every cycle (because there is more air to mix it with). It also increases the pressure of the mixture before the compression stroke begins, allowing a higher final pressure for an engine with the same compression ratio. These higher pressures, and resulting higher temperatures, lead to a higher risk of knock, but this increase in temperature is countered by the cooling effects of the injected ethanol, and does not actually cause premature combustion.

Individually, these two technologies offer only marginal increases in the performance of a standard gasoline engine. However, when combined in the way that they have been, they offer very significant improvements. By finding better ways to combine carefully regulated direct injection of ethanol (and, in point of fact, their design directly injects the gasoline, as well) with a turbocharger, they have achieved remarkable results that a year ago would have been unthinkable with what most would have thought of as technology well past its prime.

The article claims that with this combination, these standard gasoline engines can achieve fuel efficiencies that rival those of true gas-electric hybrid cars. Unlike hybrids, however, this technique does not require heavy and expensive batteries, and can run a car directly off of the engine, rather than relying on electric motors to drive the wheels. The result is a lower overall cost. With price being ranked high among the obstacles to widespread adoption of hybrid vehicles, this is an appealing prospect. Of course, the technique could also be applied to the gasoline engines already in hybrid cars, leading to even greater improvements in fuel economy.

While I’m excited to see clever people finding new and better ways to use existing technology, I’m not sure what to think about this development as a whole. I love the aspect of bright technologists making these kinds of late-stage breakthroughs. I think engineers do some amazing things, and it makes me feel good to see them find new life in something that others were ready to do away with.

On the flip side, I worry about the effect that this kind of development will have on other exciting research currently being funded by the major automotive firms. If these big, established players see that they can adopt a comparatively simple alteration to an existing technology, rather than sinking millions into the development of risky new technologies, I find it hard to believe that those research programs are going to remain funded for very much longer. They are much more likely to stick with what they are used to, and to stick with it for as long as they think they possibly can.

This really worries me, because as large as the improvements this technique purports to offer might be, even breakthroughs of this type can only extend the lifespan of the reciprocating piston engine so far. At some point, we are going to have to find something else. There is only a finite quantity of oil in the world. Burning it slower is good news, but sooner or later, it will run out. Or, in reality, supplies will shrink to the point that it is no longer an economically viable source of energy.

I’m thrilled about the prospect of building cars with similar designs to current ones, and at similar costs, but which consume considerably less gas to do the same job. The potential benefits for consumers, the environment, and the stability of Western economies are manifest. If they result in a decades-long delay in the development of the next generation of technologies that will some day replace the gasoline engine (and they very well could), that is a source of significant concern to me.

Notes:
[1] Many gasoline companies add a variety of other ingredients to high-grade gasoline that do things like help clean out your engine, which is why it might sometimes be recommended to put a tank of premium into your regular car. I make no claims to expertise about the types of additives that might be on offer at your local gas station, and I encourage you to consult with a trained mechanic before making any drastic changes to your fueling habits. Short version: If your car explodes, my lawyer doesn’t want to hear from you.

[2] In the interests of clarity and brevity, this is a simplified explanation. I make no claims to vast expertise on the subject of hydrocarbon structures. Both octane and heptane have a wide variety of isomers, each with slightly different properties, and gasoline derived from crude oil contains varying quantities of both molecules and their various isomers in widely divergent proportion based on a huge variety of factors. You are highly encouraged to consult with a trained petrochemical specialist before making any important choices in this regard. Short version: If you fail your organic chemistry exam, my lawyer doesn’t want to hear from you.

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2 Responses to Can’t stand the heat? Get out of the engine.

  1. goshawk says:

    Mark,
    This is a very interesting, and truly Geeky, post. I wonder what the cost of the high pressure injection for the alcohol and how this system ends up costwise compared to a hybrid system. With either system you have the opportunity to get more power out of a smaller engine and thereby reduce fuel use and emissions. You also have the opportunity to keep the bigger engine and just get more horsepower. I am disappointed, but not surprised, to see newer hybrid cars going in the direction of more power rather than better efficiency. The car makers say this is what the consumers want and they have no choice but to give it to them. I am not sure I completely believe that argument; but, if it is true, it would argue for higher gas prices to make consumers opt for efficiency over power. At any rate, exploring all the possibilities is the right thing to do and this was one possibility I was unaware of. Thanks for bringing to my attention.

  2. Mark says:

    Goshawk,

    According to the article, the cost to produce an engine of this type is considerably less than that of a true gas-electric hybrid system of equivalent performance (and given that it also claims equivalent efficiencies for engines of comparable performance, this makes for a reasonably fair measure, if you accept their claims). The system requires that you carry a separate tank of ethanol on-board, but this is used in small enough quantities that you’d need to fill it about as often as you changed the oil. A gallon of ethanol costs about the same or a little less, at current prices, as a gallon of gasoline. The article doesn’t specify the size of the ethanol tank that would be required to last the several months between oil changes, but regardless, we are talking about a single-digit number of dollars per month.

    It is my understanding that the appeal of this system to consumers is that it offers a significant advantage in cost. The appeal to manufacturers is that it allows them to continue to use a long-proven technology, and divorce themselves from the not inconsiderable risk of developing new technologies to replace it. Direct injection systems (as compared to port injectors) are not significantly more expensive to manufacture, so once the initial cost of retooling an engine plant to build engines with a different design was past, this part does not add much to the overall cost of a finished engine. Adding a turbocharger is a cost, certainly, but given that I can go out and add one to my existing engine as an aftermarket part for a few hundred dollars, I don’t suspect that if it were integrated into the production process for a mass-market engine that this would represent a huge increase in cost, either.

    I certainly grant you that the types of choices that some people make about their hybrid vehicles are regrettable. This is, however, the risk you run. By developing technologies that allow people to make smarter, more responsible choices, we also have the side effect of simply making their irresponsible choices less harmful in the long run. I guess it really depends on how you look at it. If you assume that people are willing to accept some set fuel economy (let’s say 30mpg, for the sake of example), the goal should be to give them the most power you can for that same quantity of gas. If, on the other hand, people demand that they have at least 250 horsepower at their command, and you are able to build an engine that can accomplish that at 30mpg instead of 15mpg, you’ve done a great thing.

    By the same token, if I, as a consumer, am willing to give up the ability to accelerate so hard it tears the tires off the rims, this kind of technology (either hybrid or direct-injected, turbocharged) allows a car builder to use a much smaller engine. Car builders have a point, to some extent, in that they can sell huge, powerful cars because people are willing to buy them. Then again, they make a much higher margin on large, powerful cars, so they are much more invested in building them and still squeaking by beneath government-mandated fuel economy standards than they are in building small, green cars. At the end of the day, auto companies can and likely should be doing more to promote greener cars, but we as consumers are the ones who have to make the smart choices that will make building green cars economically viable.

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