In the market, diesel engines are not widely used in family cars, but in The European and American countries, diesel engines are very popular among consumers mainly because of diesel engines efficiency and excellent energy saving .
Compression Ratio
A diesel engine will have a higher compression ratio than a gasoline engine, while an internal combustion engine will have a higher compression ratio and will be more efficient in its work.
Diesel engine is higher than gasoline engine compression ratio of the reason is that only in the compressed air when diesel engine compression, and even has a high compression ratio, also won’t produce ahead of the situation of the deflagration, and diesel engine depend on the temperature of the compressed air itself will fuel injection entered moment light, so need to compressed air to the diesel engine of diesel combustion temperature of above.
Gasoline engines compress the mixture, ideally to a point where the mixture is ignited by a spark plug and cannot be compressed to that high temperature, so they do not compress the mixture too much to ensure that the temperature is not too high, whereas diesel engines need to compress more to raise the temperature.
The compression ratio is directly related to the efficiency and torque of the engine, so the diesel engine with higher compression ratio can output more torque.
Combustion Rate
Gasoline engines, we all know, ignite the mixture through spark plugs. The flame spreads around the spark and burns all the mixture up, whereas diesel engines ignite almost as soon as the fuel is injected into the combustion chamber, so they can burn completely in a much shorter time.
Observe the piston movement can be found that gasoline engine piston before finish mixture combustion has been down a long distance, and diesel engine piston before fuel combustion only move a small distance, the result is that the diesel engine combustion flame spend more time in pushing the piston downward, into a more useful work and more torque.
On a gasoline engine, if the piston is still burning far from TDC, the flame can only push the piston a short distance, and if it is closer to TDC, it can push the piston a longer distance.
So gasoline engines can also maximize torque output if they have the transient combustion characteristics of diesel engines.
Cylinder Bore Rtroke Ratio
In fact, diesel engines tend to have a long stroke with a small cylinder diameter, while gasoline engines tend to have a long stroke with a large cylinder diameter with a short stroke. Since torque is the force x distance, in the application of diesel engines, there is not only a greater force, but also a longer distance (longer crankshaft crank) to amplify the force, which ultimately translates into greater torque.
For example, suppose you have two 0.6L single-cylinder engines with a cylinder diameter of 80mm and a stroke of 120mm.
The cylinder diameter of gasoline engine is 100mm, stroke is 80mm, and displacement is 0.6L, but the stroke of diesel engine is much longer.
It is worth noting that because diesel burns much faster, combustion does not occur at the optimal crankshaft Angle, so long strokes are a significant help in producing more torque.
More importantly, the diesel engine has a higher compression ratio, and it is the combination of instantaneous combustion and turbocharging that influences the high torque of the diesel engine.
A longer stroke limits the speed of the diesel engine, because if the stroke is longer, the piston will move faster at the same speed. For example, if the speed is 3000rpm, the average speed of the piston of a 120mm stroke is 12m/s, and the average speed of this piston corresponds to 4500rpm of a gasoline engine with 80mm stroke.
So the average speed of pistons in both engines is the same, but the speed of gasoline engines is much higher.
Diesel engines are better suited to turbocharging.
The higher strength of diesel components and the higher strength of cast iron cylinders, pistons, connecting rods, and crankshafts make the engine more bulky, but also able to withstand greater torque.
Another feature is the thin burning of diesel engine, diesel engine air-fuel ratio can reach now ~ 70:1, through the thin combustion, there will be more air can be used to expand to do useful work, so the efficiency of the engine will be higher, achieve the effect you need the help of the turbocharger, use the turbine pressure as much as possible into the more efficient combustion air.
Diesel engines also have lower pump losses than gasoline engines because they do not need throttle valves, or have one that almost always opens to maximum.
In contrast, gasoline engines have a much smaller air-fuel ratio range, and the boost value is not too large, because the original gasoline engines are not designed to withstand the large boost value that diesel engines do.
In general, more pressurization, more fuel equals more horsepower, and therefore more torque.
Diesel Has a High Energy Density
For the same volume of fuel, whether it’s 1L diesel or 1L gasoline, diesel USES about 10-15% more energy than gasoline, so by injecting the same amount of fuel, diesel produces more energy, which is more torque.
There was a study at the University of Wisconsin in the United States, using a mixture of diesel and gasoline to achieve a 60% thermal efficiency of the engine, but this was achieved under laboratory conditions.
This is mainly because diesel and gasoline burn at different speeds, using a manifold, then a direct injection of diesel into the cylinder, and then using diesel to ignite the gasoline. Under low loads, less gasoline is used and more diesel is used, while under high loads, the ratio of gasoline to diesel can be as high as 9:1.
The technology is low-emission at first, producing almost no nitrogen oxides, and well-blended diesel is less likely to produce particulate matter because it is pre-sprayed and mixed.
In addition, although diesel is used, this kind of engine can meet the 2010 EPA emission regulations without installing exhaust treatment devices, so it will have more advantages in exhaust treatment than traditional diesel engines.
But because you need two types of fuel, you need two sets of fuel, and that’s where the engine falls short.
At the same time, if you want to achieve 60% of the thermal efficiency, you must stop the cooling of the piston, but this is a test of the durability of the parts, but also the future mass production needs to be solved.
The advantages of this engine are also in thermal efficiency, according to understanding, the maximum thermal efficiency can reach 55%, but the form of work is different from the traditional diesel engine, this engine USES the opposite combustion chamber design, this engine is developed by Achates Power.
The earliest opposed-piston engines date back to the late 19th century and were used in ships, airplanes, submarines, tanks, trains and other vehicles. Rolls-Royce even developed opposed-piston engines for military use in the 1950s.
The pistons of opposable piston engines are different from ordinary engines in that the pistons move in opposite directions and meet at the top dead center and then respectively start to do work. This kind of engine is a two-stroke engine, which means that the pistons start to do work when they meet in the middle, so the air change stroke of a four-stroke engine is not needed.
The motor’s basic working principle is very simple, when the piston reaches the next check point, the fresh air from the air inlet below the cylinder into the cylinder, gas from the top of the cylinder after combustion exhaust discharge, after two pistons close to each other, near the midpoint before diesel can be directly sprayed into the cylinder, each cylinder has two to the installation of the nozzle.
The combustion begins by separating the pistons, which are then repeated throughout the cycle, with the upper and lower pistons each using a separate crankshaft, which is combined by gears to produce power as a shaft.
Using two crankshafts may seem complicated, but in fact the engine has no valve structure, meaning no camshafts, rockers, valves, valve springs or even cylinder heads.
For example, a 2.7-liter, 3-cylinder high-powered computer can deliver 270 horsepower and 650N•m of torque.
First of all, without the cylinder head, after the burning hot gas contact area is smaller, than the area of the combustion chamber, the contact area is reduced a lot, relative to the heat transfer to the cylinder head, it’s heat transfer to the both sides of the piston, which means that more heat is used to do work, and the whole movement distance between the two piston cylinder, which makes stroke bore ratio is large, and the movement of the piston velocity is very low, this makes the energy utilization ratio is higher.
In addition, the combustion Chambers at the top of the pistons on either side could be ideally designed so that the air and fuel would mix faster and burn faster.
The two stroke engine by the mechanical booster to control the inlet pressure, and there is a turbocharged double charging structure, air inlet and vent is designed to be the most beneficial to scavenging, vent will first open, let burning after high temperature and high pressure gas discharge, when the pressure in the cylinder is reduced, air inlet opening, fresh air into the cylinder to the rest of the waste gas discharge, into the exhaust outlet in the center of the piston to the movement when closed, there will be a small amount of gas remaining.
From the perspective of efficiency, this is the most ideal configuration. If there are only two cylinders, there will be a large vacuum period for the airflow supplying the turbine, which will reduce the overall efficiency. In particular, when the speed is low and the load is low, there will not be enough exhaust airflow to push the turbine.
If there are four cylinders, there will be too much overlap between the opening time of one cylinder and the opening time of the other cylinder, which will increase the residual gas in the cylinder, reduce the scavenging efficiency, and ultimately reduce the overall efficiency as well.
Of course, this doesn’t mean that the engine can’t use other configurations. 2, 3, 4, and 5 cylinders are ok, and Achates is also developing a performance oriented four-cylinder engine, but from an efficiency point of view, a three-cylinder engine is ideal.
When it comes to efficiency, the test data show that in a very wide range of load conditions, the engine output thermal efficiency can be in more than 40%, the maximum thermal efficiency reached 46%, cylinder heating under load test efficiency are no less than 51%, so the advantage of this engine is not only a maximum thermal efficiency is very high, and has a very wide range of speed and load under can have very high thermal efficiency.
In 2012, a 2018 SAE study reported that a three-cylinder, opposed-piston diesel engine achieved a maximum thermal efficiency of 55%.
At the 2017 Detroit Auto Show, nine major manufacturers were partners in the development of the engine, and only one has started experimenting with the opposed-piston engine.
However, as it is a two-stroke engine, it will have an obvious disadvantage in environmental protection, because the oil of two-stroke engine is involved in combustion, and under the increasingly strict environmental regulations, it will inevitably suffer a devastating blow.
In addition, since there is no valve mechanism, the position and height of inlet and exhaust port will affect the phase of inlet and exhaust of the engine, and directly affect the performance of the engine. Meanwhile, different variable valve technologies are available, so it is difficult to meet the efficiency and power performance in any working condition.
Conclusion
To sum up, diesel engine is superior to gasoline engine, but if diesel engine can be popularized, oil quality must be improved first. Only with good supporting facilities, engine can play its strength.
Hybrid combustion engine has excellent thermal efficiency performance, but because it needs two fuel filling systems, it is not conducive to the use of mass production models.
The same is true for the opposite piston engine, which has high thermal efficiency. However, whether the vibration of the engine, the output and emission of each working condition can meet the standard of mass production still needs the manufacturers to continue to work hard.
