Through this site, we previously reported that, when CNG buses were first being introduced, an LPG bus was also put on trial. At the time, vehicles were fitted with a German MAN mixer-type LPG engine, but reports cited insufficient output and various issues that made regular operation difficult. Those shortcomings underscored the need for a domestically developed LPG engine—and as of 2003, a Korean-made LPG engine has arrived and is drawing strong attention.

The newly developed domestic LPG engine is a large-displacement 300-horsepower class unit using an LPLi system, which boosts output and significantly reduces emissions. While CNG buses are being deployed in major cities, regions without CNG infrastructure—smaller cities or mountainous inland areas—could adopt LPG buses as a practical alternative. In this feature, we take a closer look at the new Korean bus-spec LPG engine.

Low-Emission, Large LPLi-Type LPG Engine

Reported by: BusLife
Materials: KIMM LPG Engine R&D Program (http://lpli.kimm.re.kr/)
Date Posted: September 19, 2003

The vehicles above were among the first attempts at LPG city buses. They mounted MAN’s 240-horsepower LPG engine and a ZF automatic transmission. With most city buses of the era running ~290-horsepower diesels, 240 hp proved too little. The imported, mixer-type engine offered no path to higher output, and opinions were widespread that it wasn’t well suited to Korean duty cycles.

Given today’s rollout of natural gas buses, some may ask whether LPG buses are even necessary. National policy has driven CNG deployment, but many parts of Korea still lack natural gas infrastructure—hence the pressing need for an LPG bus option.

Currently supplied domestic LPG engines are mixer-type and cannot truly be called low-emission. We therefore need to convert from mixer systems to LPLi—direct liquid-phase LPG injection. Overseas, LPLi engines are already developed and commercialized. With more than a million LPG automobiles on Korean roads, developing a low-emission LPLi engine is essential. Because of technological limits, legacy LPG engines emitted combined hydrocarbons and NOx at levels comparable to gasoline engines.

Comparison of exhaust performance: Mixer vs. Korean LPLi (2002, Korea)

The figures clearly show that LPLi slashes exhaust emissions versus mixer systems. Beyond bus deployment, LPLi development is vital to advancing domestic engine technology. So what exactly is LPLi?

LPLi stands for Liquid Phase LPG Injection. It is regarded as the third generation of LPG fuel-delivery systems.

Gasoline engines evolved from carburetors → single-point injection → multi-point injection → direct injection.

LPG engines are expected to evolve from carburetors → mixer systems → multi-point injection → direct injection.

1st Gen: Carburetor System + Open-loop control (1970s) 2nd Gen: Feedback Mixer System + Closed-loop lambda control (1980s) 3rd Gen: LPLi System (Multi-point Injection) (1995) 4th Gen: Direct Injection (Otto/Diesel concepts) (under research)

That’s the lineage. Advanced markets developed LPLi in 1995; Korea has only now reached that milestone.

So what separates 2nd-gen mixer systems from 3rd-gen LPLi?

The big gains are higher output and lower emissions. Power rises about 15% versus mixer types, achieving gasoline-like performance, and the system meets EURO-IV emission limits—delivering true low-emission operation.

Comparison of exhaust performance: Mixer vs. Korean LPLi (2002, Korea)

 Let’s revisit the chart above.

CO, HC, and NOx are all dramatically lower with LPLi than with mixer types.

KIMM’s LPG Engine R&D Program has fitted the LPLi system to a Trajet XG for trial operation and study.

They’ve also developed a large engine suitable for buses, completed pilot operation, and plan deployment in Jeju.

Key specifications of the newly developed bus LPG engine are as follows.

Engine type	Turbocharger (TCI)
	LPG engine
Combustion	SI lean-burn
Bore × Stroke	130 × 140 mm
Displacement	11.15 liters
Cylinders	Inline-6
Compression ratio	9.3 : 1
Fuel system	LPLi System

Core technologies in this engine include:

1) Low-viscosity fuel compression & pumping, 2) Liquid-phase retention for low-boiling fuel, 3) Anti-icing, 4) Injectors for low-viscosity fuel, 5) Stratified combustion of fuel.

In short, the technical hurdles of injecting a gaseous fuel in liquid phase have been addressed.

Ministry of Environment certification data (June 19, 2002)

From the table, you can gauge the exhaust levels of the new LPG engine. In short, LPLi offers clear advantages over mixer systems.

Specifications of the newly developed Korean LPG bus are as follows.

The bus shown at right is the newly developed LPG model. The body is Hyundai’s Super Aero City, powered by the new KL6i inline-6 turbo-intercooled LPG engine. The bus currently runs all day in Daejeon—from around 6 a.m. until evening. For accurate testing, approximately 3.5 tons of cement bags are loaded onboard. Testing empty would mask issues that appear under high passenger loads, so the vehicle is run under payload equivalent to about 54 riders (assuming 65 kg per person). Routes were chosen to include both urban and interurban segments, and not only level ground but a steep climb over Mati-jae Pass to validate gradeability.

On September 3, 2003, we conducted a test drive of the LPG bus.

We took the wheel for roughly two hours on actual service streets. With about 3.5 tons aboard, the bus felt satisfyingly planted—the springs were visibly compressed. Starting in 2nd gear, launch was smoother than expected with no bogging. CNG buses often need heavy half-clutch work when starting in 2nd due to low-rpm torque, overheating and wearing the clutch. By contrast, this LPG powertrain performed nearly at diesel levels.

Only a brief half-clutch was needed before clean take-off.

As shown at right, about 3.5 tons of cement are loaded inside, and a computer is connected for test logging and analysis.

In real time, the system records running state and acceleration, checking whether the engine behaves normally in each scenario. The bus operates daily for extended hours, with all data collected and analyzed.

Metrics such as acceleration behavior, temperatures, and resonance conditions are logged and displayed.

Proper analysis helps improve durability and performance—and also trace sources of noise for NVH refinement.

With strong low-end torque, launch feels better than a CNG bus and not far off a diesel. Though current calibration is 290 hp, the engine was developed for 300+ hp, so it never felt lacking. With 3.5 tons onboard, we climbed the Mati-jae grade in 4th gear; unladen, it reportedly manages the same climb in 5th. When we wanted to overtake, throttle input translated to ample acceleration. One note: because acceleration builds quickly from rest, you tend to row through to 4th—much like a contemporary diesel bus.

We tried to mimic city-bus driving for two straight hours; the brisk acceleration really impressed. Being a turbo-intercooled LPLi engine, when you lift at the target rpm after a deep throttle press, revs can flare about 100 rpm further depending on the boost state. During hard acceleration, allow for turbo-charger time lag and ease off slightly before the rpm alarm—good practice for clean shifts.

We’ll also check whether turbo lag is as prominent on typical diesel buses. It’s arguably a quirk, but the upside is you don’t need the so-called “blipping” (revving in neutral) between shifts. Most drivers won’t even notice—it’s minor.

When we drove a Hyundai Super Truck 11.5-ton turbo-intercooler in the past, a sudden deep stab of throttle produced a ~0.5-second lag before the surge; by comparison, this LPLi LPG engine felt no worse in initial response.

It would be interesting to run a straight-line comparison—diesel vs. CNG vs. LPG buses—over ~1 km, measuring at 50 m, 100 m, 500 m, and 1 km to map each engine’s character.

Today, CNG buses are being deployed under national policy. In that context, one might ask: why develop a large LPG engine for buses? Years ago, when MAN’s LPG engine was imported, TEAM BusMania took a skeptical view of LPG buses. But CNG rollout faces headwinds—resistance to new filling stations and, even where stations exist, growing fleets are straining capacity. Some may see CNG and LPG development as duplicated investment, and if pitted head-to-head, both could lose.

So how should we interpret this LPG bus project now?

Mountainous Gangwon, parts of the interior, and Jeju lack natural gas infrastructure. Some inland areas rely on mobile CNG filling. Building gas infrastructure demands heavy budgets; you can’t invest lightly, and laying pipelines in Gangwon or Jeju is no small task. In such cases, CNG and LPG can share the load. Regions without CNG currently run diesel buses. Even with catalyst filters, overdue maintenance can worsen emissions, and the filters aren’t cheap. Since both modern CNG and LPG engines are low-emission, deploying this new LPG bus where CNG isn’t feasible makes sense.

Given the tech progress, we see no reason to oppose LPG buses now. Rather than running diesels for 5–10 years while waiting for CNG infrastructure, adopting LPG buses strikes us as the wiser course.

The government has invested heavily to spread CNG buses. Suggesting LPG buses in that context could look like undermining a carefully built policy tower. To be clear, we’re not advocating a wholesale shift to LPG. But in areas where CNG operation is not viable, replacing diesel city buses with LPG units could cut worsening air pollution and, in partnership with CNG, better protect air quality.

Alongside that, fuel-price policy for CNG and LPG should diversify: vehicles using eco-friendly, low-emission engines deserve reduced environmental charges to ease operator burdens. As technology advances, these buses will emit less than many passenger cars—benefits should follow.

For too long, buses—belching black soot—were seen as prime polluters. We look forward to the day when buses are guardians of clean air, and gasoline cars are recognized as the bigger culprits.