Shulou(Shulou.com)11/24 Report--

Original title: "Oil truck, oil truck, where is your efficiency?" "

Efficiency is life, and it is always established for civil power systems. Today, the replacement of internal combustion engines by electric motors appears to be a qualitative change of "burning oil but not burning oil", but inside it is still a continuation of "those who are less difficult to improve efficiency (those who begin to have the ability) to supplement / replace those who are more difficult to improve efficiency".

We know that with the development of automotive gasoline engines today, the peak thermal efficiency of measuring products is just over 43%; even for the next generation of models under development, the goal is also "just" to try to reach 50% of the cost-while electric power is often more than 90% of the ultra-high efficiency effortlessly.

So why is the internal combustion engine so inefficient by comparison? Where is more than half of the energy wasted?

In each part, it is calculated that the thermal efficiency of the internal combustion engine currently stops at 40-50%, and more than half of the remaining energy has a definite "destination".

According to the more commonly used and thicker division, mainly exhaust loss (exhaust loss), cooling loss (cooling loss), friction / mechanical loss (friction / machanical loss) and pumping loss (pumping loss), the remaining proportion is the so-called thermal efficiency.

Among them, friction / mechanical loss is easier to understand, and exhaust loss refers to the heat energy contained in the exhaust, needless to say. The largest proportion of cooling loss in the picture refers to the heat lost in order to keep the internal temperature of the engine not too high, or the extra heat needed to maintain normal combustion.

Although it is our usual understanding that the engine needs to dissipate heat, high efficiency actually requires converting as much fuel chemical energy as possible into kinetic energy rather than heat. The ideal condition is that there is no need to dissipate heat at all, whether the temperature in the cylinder exceeds the need for combustion or the heat emitted by the engine as a whole comes from the fuel consumed.

For a four-stroke engine, the piston moves up and down four times (two back and forth) and only once is driven by fuel combustion. Then the intake stroke piston down suction, exhaust stroke piston upward exhaust, in fact, need to overcome the external atmospheric pressure resistance (equivalent to inward pumping and outward pumping), this is the exhaust loss.

Whenever a car company launches a new internal combustion engine product, most of the improvement measures in the copywriting can be put into place. The Atkinson / Miller cycle reduces the exhaust temperature and pump loss, and a series of measures to increase the thinning of the mixture reduce the combustion temperature and reduce the cooling loss and exhaust loss.

The peak value is not equal to all. It should be noted that the proportion of these parts will also change when the internal combustion engine operates under different operating conditions. That is to say, the proportion of several color blocks in the above picture changes with the horizontal (load) movement, and the increase and decrease direction / rate is not necessarily synchronized.

This is not difficult to understand, for example, the friction loss will decrease with the increase of engine load, because there will be mechanical friction even if the throttle is sliding, so the friction loss will naturally account for a larger proportion at low load; with the increase of load (increasing throttle), the amount of friction loss will increase but the proportion will decrease, that is, the drag on thermal efficiency will be reduced.

The increase / decrease of each type of loss part is not synchronized with the increase of load, and the result is that the thermal efficiency of the "remaining" is in a less linear change. Reflected in the BSFC specific fuel consumption diagram, which is necessary to examine thermal efficiency, it is the zigzag contour of thermal efficiency.

It is also because the "temper" of all kinds of loss is different under different working conditions, in order to improve the comprehensive thermal efficiency of the internal combustion engine, it is necessary to consider the optimal balance of each typical operating point. For example, if the friction loss is optimized with great effort and high cost, the efficiency improvement is more obvious for low load conditions, but the effect may be limited under high load conditions.

At this time, it should be noted that the so-called thermal efficiency of internal combustion engine is not a global value, but only represents a maximum value. If you look closely, you will find that there is always the word "highest" or "peak" in front of the "thermal efficiency 41% 42% 43%" of the promotional copy.

For example, Toyota's famous A25A series 2.5L engine has peak thermal efficiency of 40% and 41% respectively for fuel version and hybrid version. The former reaches 40% at the operating point of about 2500rpm and 160 N ·m (at the red star below), while the latter has a difference of only 1%, but the efficiency distribution is completely different. For example, the efficiency area greater than 38% is one circle larger than the former.

The difference is not just that the maximum efficiency is 1% less than the "40% thermal efficiency", nor does it mean that vehicles can always burn gasoline at 40% efficiency, because fuel vehicles or even hybrid cars, it is unlikely that the engine can always run at the "speed-torque" of the best efficiency point, as long as there is a deviation from the efficiency, the overall efficiency is bound to be lower than the peak thermal efficiency.

The efficiency of synthesis depends not only on the peak and contour distribution given by the BSFC diagram, but also on the method or standard of measuring this "synthesis". Just like different driving habits, different acceleration and deceleration routes driving the same car, fuel and electricity consumption must be high and low.

If it is only to increase the peak point efficiency value, it may not necessarily significantly improve energy consumption performance (but high peaks do tend to correspond to higher overall efficiency). In addition to improving the peak value of thermal efficiency, we can also try to expand the area of relatively efficient area and the most efficient point closer to the most commonly used operating conditions, which will be helpful to the actual application effect.

Seeing the upper limit, the world has removed some very biased start-up concept products, and today the goal of the highest thermal efficiency is 56% of Mazda's next-generation Skyactiv-3 (provisional name). Even if F1 does not hesitate to cost and add various energy recovery buff, it still hovers around 50% in the end.

In addition to the final peak thermal efficiency, "indicated thermal efficiency" is also beginning to be mentioned. The so-called indicated thermal efficiency can be understood as the work efficiency of pure combustion in a narrow sense, which only looks at the single cylinder fixed on the bench, does not include any accessories, to the piston without going to the crankshaft.

At the beginning of 2021, Nissan announced that it had achieved 50% bench thermal efficiency, or indicated thermal efficiency. The Raytheon hybrid engine released by Geely at the end of the year claims to indicate a thermal efficiency of 52.5%, while the next-generation model has a target of 57%.

It indicates that thermal efficiency is the absolute ceiling of actual thermal efficiency, and there are inevitable gaps and gaps downward, even though today's new-generation internal combustion engines have made every effort to electrify and de-attach to reduce the "landing loss" between the two.

Internal combustion engine as a heat engine, according to the Carnot cycle efficiency formula combined with today's basic science level, the limit of thermal efficiency is roughly a little more than 60%. Naturally, when the new generation of internal combustion engines break through 40% relatively quickly and gradually approach 50%, and the indicated thermal efficiency is getting closer and closer to 60%, the bottleneck becomes more and more prominent.

It is the peak thermal efficiency of the internal combustion engine that approaches the current technical limit in theory step by step, and it becomes more and more idealistic simply to improve the thermal efficiency to reduce the level of energy consumption. and the means such as hybrid technology, which can make full use of the highest efficiency area, has become the main theme of today's era, until it moves further towards pure electricity.

As for where the electricity used by the efficient motor comes from, and whether the battery will hinder the high efficiency of the motor, which gains or loses when the drive and energy storage are matched separately, it is another more complicated and grand topic.

This article comes from the official account of Wechat: autocarweekly (ID:autocarweekly), article: Aohu

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